Faculty of Bioscience Engineering. Academic year

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1 Faculty of Bioscience Engineering Academic year Evaluating rainwater harvesting on watershed level in the semi-arid zone of Chile - Evaluatie van watercaptatietechnieken op stroomgebiedsniveau in de semi-aride zone van Chili Lynn Verstrepen Promotor: Prof. dr. ir. Wim CORNELIS Tutor: dr. ir. Koen VERBIST Masterproef voorgedragen tot het behalen van de graad van Master in de Bio-ingenieurswetenschappen: Milieutechnologie

2 Copyright De auteur en begeleiders geven de toelating dit project voor consultatie beschikbaar te stellen en delen ervan te kopiëren voor persoonlijk gebruik. Elk ander gebruik valt onder de beperkingen van het auteursrecht, in het bijzonder met betrekking tot de verplichting uitdrukkelijk de bron te vermelden bij het aanhalen van resultaten uit dit project. The author and promoters give the permission to use this project for consultation and to copy parts of it for personal use. Every other use is subjected to the copyright laws, more specifically the source must be extensively specified when using from this thesis. De promotor, De begeleider, Prof. dr. ir. Wim Cornelis dr. ir. Koen Verbist De auteur, Lynn Verstrepen 25 augustus 2011 I

3 Acknowledgement The amazing experience of this thesis and the experimental work in Chile would never have been possible without the help and support from several people. Therefore I would like to thank these people on this page. I am very grateful to: My promoter, Prof. dr. ir. Wim Cornelis for correcting my text, for giving me the opportunity to perform this thesis and the experimental work of it in Chile. Dr. ir. Koen Verbist for teaching me to do the experimental work and to steer everything in a good direction, for correcting my text and giving me valuable opinions on it and for helping me with all the problems that occurred during this thesis (from getting stuck in a ditch with the car in Chile to the problems with the model). Maarten Volckaert for helping me with the analysis of the soil samples in the laboratory in Belgium. Mauricio Lemus, Edmundo Gonzalez, Jorge Nuñez, Juan and Yolanda for the help in Chile. CAZALAC (Zonas Áridas y Semiáridas de América Latina y El Caribe) without which the field work in Chile in all its aspects would not have been possible and CONAF (Chile's National Forestry Corporation) for the use of the measuring equipment. The Faculty of Bioscience Engineering for the financial support. Isabel, for helping me with the experimental work in Chile and the support whenever there were problems, for the company and the unforgettable trip. José and family, for letting me stay in their homes when arriving, traveling and leaving Chile and for teaching me about the beautiful culture of Chile. Marissa, for helping me to learn Spanish and giving useful tips about Chile. Mathias, for reading my thesis and correcting the spelling. My mother, Karine, for granting me the opportunity to study and supporting me in everything I do. Last, Tom, the one person who is always there for me and helped me through the difficult times. Every time when something went wrong or when I was struggling, he convinced me that in the end everything would be worth it And guess what He is right. Lynn Verstrepen, August 2011 II

4 Contents Chapter 1 Introduction 1 Chapter 2 Description of the study area Chile Socio-economic situation Geography Climate Alto Loica Geography Climate Erosion control and afforestation in watersheds in the semi-arid region of Chile 6 Chapter 3 Literature review Land degradation Surface runoff The surface runoff process Factors affecting runoff and erosion Water harvesting techniques Classification of water harvesting techniques In situ RWH Micro-catchment system Macro-catchment system The infiltration furrow 14 Chapter 4 Materials and methods HydroGeoSphere Main processes Subsurface flow Surface flow Flow coupling Flow boundary conditions Grid builder Input and output files Input files Output files PEST Model Calibration Rainfall simulation Set-up of the rainfall simulator Time Domain Reflectrometry Field measurements Guelph permeameter 35 III

5 4.3.2 Tension disk infiltrometry Soil moisture characteristics Statistical analysis 39 Chapter 5 Evaluation of the effect of the infiltration trenches in Alto Loica Evaluation based upon discharge measurements Evaluation based upon field measurements Guelph permeameter Tension disk infiltrometry Soil moisture characteristics Evaluation based upon modeling Calibration Results calibration process on a rainfall plot of 1 x 1 meter Control of capturing the dynamic responses Calibration on watershed scale Evaluation of the infiltration trenches based upon modeling 56 Chapter 6 Conclusion and discussion Evaluation of infiltration trenches applied in Quillay Evaluation of infiltration furrows via rainfall and discharge measurements Evaluation of infiltration furrows via field measurements Evaluation of infiltration furrows via modeling Problems and possibilities related to HydroGeoSphere Problems Opportunities 63 Chapter 7 Nederlandse samenvatting (Summary in Dutch) Inleiding Beschrijving van het studiegebied Chili Alto Loica Literatuurstudie Landdegradatie en oppervlakkige afstroming Watercaptatietechnieken Classificatie Infiltratiegreppels Materiaal en methode HydroGeoSphere Belangrijkste processen Grid builder Input en output bestanden PEST Model kalibratie Regenvalsimulatie 71 IV

6 7.4.3 Veldmetingen Guelph permeameter Tension disk infiltrometer Bodemvochtkarakteristieken Statistische analyse Effectevaluatie van infiltratiegreppels te Alto Loica Evaluatie op stroomgebiedsniveau gebaseerd op afstromingsmetingen Evaluatie op stroomgebiedsniveau gebaseerd op veldmetingen Guelph permeameter Tension disk infiltrometer Bodemvochtkarakteristieken Evaluatie op stroomgebiedsniveau gebaseerd op modellering Kalibratie Evaluatie van de infiltratiegreppels gebaseerd op modellering Besluit 79 References 81 Appendices 86 A Grok-files 86 A.1 Grok-file of catchment Testigo 86 A.2 Grok-file of catchment Quillay 90 B Etprops-files 95 A.1 Etprops-file of catchment Testigo 95 A.2 Etprops-file of catchment Quillay 96 C Statistical analysis 99 C.1 Statistical tests with Guelph permeameter 99 C.1.1 Kolmogorov-Smirnov Test 99 C.1.2 Independent two-sample T-test 99 C.1.3 One-way ANOVA 101 C.2 Statistical tests with tension disk infiltrometry 102 C.2.1 Kolmogorov-Smirnov Test 102 C.2.2 Independent two-sample T-test 103 C.3 Statistical tests with soil samples 106 C.3.1 Kolmogorov-Smirnov Test 106 C.3.2 Independent two-sample T-test 106 C.3.3 One-way ANOVA 107 V

7 Chapter 1 Introduction Physical movement of soil can lead to soil degradation through the process of soil erosion. The most important erosion agents are wind and water, but water erosion is the most significant agricultural problem throughout Chile. Almost every area, where food is produced and crops are cultivated, has to deal with this problem. Erosion rates in some areas of Chile are greater than 100 tonnes per hectare each year. Approximately 25.4% of continental Chile is subject to erosion and the erosion affects more than 60% of the total usable land (Ellies, 2000). Environmental fragility, topographic condition and the use of natural resources have generated different levels of soil degradation. This manifested in the loss of nutrients, plant cover and depressed crop production. At the beginning of the decade of the 60s, circa 60% of soils in the Coastal Mountain Range of Chile showed visible signs of surface erosion. In severe cases of erosion, gullies of variable depth were formed (IUCN, 2001). Arid and semi-arid zones are characterized by a notable deficiency in water availability for plant growth. On the other hand, the soil is quickly saturated because precipitation often comes in the form of short bursts of high intensity rainfall. This promotes soil erosion, flash floods and in extreme cases even mud flows. Runoff harvesting techniques such as infiltration trenches are often applied to increase rainwater infiltration and water retention on steep slopes. They are also an erosion control measure to reduce land degradation hazards (Verbist et al., 2009). Various demonstration projects were realized in Chile, some even dating back to A specific demonstration project was launched by the Government of Japan and that of Chile. The overall objective of this erosion control and afforestation project was to promote sustainable development through conservation and restoration of soils and waters (CONAF and JICA, 1998). Although various projects were realized, few efforts were made to quantify the effect of water harvesting techniques. Therefore, this thesis investigates the effect of infiltration trenches on the discharge response of the watershed, through increased water retention. A combination of detailed field measurements and modeling with the HydroGeoSphere software package was used to visualize the effect of the infiltration trenches. This was done by evaluating data available from measurements taken during the cooperation between CONAF (National Forestry Corporation) and JICA (International Cooperation Agency of Japan) and by using these data to calibrate the hydrological 3D model. 1

8 A general description of Chile and the study area of Alto Loica will be given in the second chapter of this thesis, in terms of socio-economic situation, geography and climate. A brief summary about a major project between CONAF and JICA will be included at the end of the second chapter. The third chapter comprises a literature review on soil degradation and erosion. A solution to these problems, namely water harvesting techniques with particular attention to the infiltration furrows, will also be discussed. In the fourth chapter the materials and the methods that were applied throughout the thesis will be explained. In the first section, the HydroGeoSphere software package will be explained which was used to visualize the effect of the infiltration trenches. In the second section of this chapter the PEST software which was used for automated calibration will be outlined. In the third section, the materials used for the field measurements will be described. In the fifth chapter, the evaluation of the infiltration trenches is performed. First, the effect is evaluated without modeling and is only based on the analysis of measurements of discharge due to rain. In the second section the results of the field measurements are evaluated. In the third section of this chapter PEST was used for automated calibration. Afterwards, the evaluation of the effect of the infiltration trenches is performed using HydroGeoSphere. In chapter six, conclusions are given. Finally, a brief summary of this thesis is given in Dutch at the end of this thesis. 2

9 Chapter 2 Description of the study area 2.1 Chile Socio-economic situation The total population of Chile amounts to with a population growth rate of 0.881% (CIA, 2010). 86% of the people live in urban areas with 40% living in the capital of Santiago (LOC, 2010). In Chile there are 2.6 million people employed and 13.9% is employed in the forestry and agriculture sector (U.S. Department of State, 2010) Geography Chile is a long (4270 km) country in South America and has an average width of 177 km (LOC, 2010). The surface area amounts to km 2 but only 2.62% of the land area is arable (CIA, 2010). Chile has since 2007 been divided into 15 administrative regions (figure 2.1). Each region (except the metropolitan region of Santiago) begins with a Roman numeral followed by a name (CONAF, 2010). Figure 2.1: Map of the 15 administrative regions of Chile (CONAF, 2010) 3

10 Chile has a highly varying geographical character. It varies from north to south, as well as from west to east. The north is characterized by one of the driest deserts, namely the Atacama Desert. More towards the south, the land falls away and the region between mountains and ocean fades into the baffling archipelagic maze that terminates in Chilean Patagonia (U.S. Department of State, 2010) Climate Chile's climate is as diverse as its geography. It comprises a wide range of weather conditions across a large geographic scale, making generalizations difficult. Figure 2.2 and table 2.1 show the different soil moisture regimes and their characteristics in Chile. In general, 60% of the land has a xeric to sub humid climate and therefore more than half of Chile is in a vulnerable state (Verbist et al., 2010). Figure 2.2: Hydric regimes in Chile (Verbist et al., 2010) 4

11 Table 2.1: The different soil moisture regimes in Chile (Verbist et al., 2010) Moisture regimes Percent of total area Number of dry months Annual precipitation (mm) Xeric 18% Hyper arid 8% Arid 13% Semi-arid 13% Sub humid 8% Humid 9% Hyper humid 8% Hydric 9% Hyper hydric 15% Alto Loica Geography Alto Loica is located in the metropolitan region (about 120 km southwest of Santiago), in the province of Melipilla and the community of San Pedro. The project site is located at 34 latitude and 71 longitude, which is close to the borderline of the 6th region (figure 2.3) and is hectares. The area consists of mild hills of 200 to 350 m elevation. The slopes of the hills range from 5 to 15 degrees. Sheet erosion can be observed in 63% of the area and gully erosion is present in 24%. The region has been used for wheat cultivation and grazing after deforestation. The surface soils have a sandy clay texture (Ap-horizon with little organic matter). The saturated hydraulic conductivity of the A- and B-horizon is respectively 10-3 cm s -1 and cm s -1. The area is underlain by weathered granite. Outcrops of bedrock could only be seen in the gullied areas (Fujieda and Vargas, 2005) Climate Figure 2.3: Location of Alto Loica (Fujieda and Vargas, 2005) The climate is known as a Mediterranean climate with precipitation in winter (Tokugawa and Vargas, 1996). The mean annual rainfall for the period 1961 to 1991 was mm with a standard deviation of mm. Circa 88% of the annual rainfall occurred from May to September. The maximum temperature was 32.2 degrees in January. The minimum temperature on the other hand was 3.4 degrees in July. The mean annual temperature was 15.3 degrees. The catchment may be under dry 5

12 conditions except during the winter rainy season (Fujieda and Vargas, 2005). The Mediterranean climate is one in which winter rainfall is more than three times the summer rainfall. This strong contrast results in root zone drying of the soil during the summer, often for several months (Yaalon, 1997). The wind is also a factor which plays an important role in the dryness of the region. The wind blows during fall from 11 a.m. till 8 p.m. in south-east direction and the mean velocity is 4 m s -1 but can go up to m s -1 (CONAF and JICA, 1995). According to the Aridity Index AI [-] adopted by UNEP (United Nations Environment Programme), the study area can be categorized as semi-arid (Middleton and Thomas, 1997). The AI is a numerical indicator of the degree of dryness of the climate at a given location and is defined as: = (2.1) with P [m] the mean annual precipitation and ET 0 [m] the mean annual potential evapotranspiration. 2.3 Erosion control and afforestation in watersheds in the semi-arid region of Chile Through the National Forestry Corporation (CONAF) and the International Cooperation Agency of Japan (JICA), the Government of Japan and that of Chile launched a major engineering project. This project was called Erosion control and afforestation in watersheds in the semi-arid region of Chile. The erosion control and afforestation project began in March 1993 and took 6 years. The project took place in three demonstration areas, namely sector Yerba Loca in the community Lo Barnechea (Metropolitan region), sector Las Cañas in Illapel (4 th Region) and sector Alto Loica in San Pedro. The goal of the project was to improve the quality of life for the people and the environment of the semi-arid areas of Chile, through technological development and demonstration of erosion control issues. The overall objective was to promote sustainable development through conservation and restoration of soils and waters. The following methodology was used (CONAF and JICA, 1998): Identification of the degraded areas of the catchment Assessing the state of degradation of the units of the basin Mapping the degraded areas Setting priorities and criteria for intervention Selection of erosion control treatments Implementation, monitoring and maintenance of the erosion control treatments Especially the last part is of interest to this research. To achieve a suitable erosion control treatment, three micro-basins were set up in the sector Alto Loica (CONAF and JICA, 1995). In these micro-basins water harvesting techniques such as the infiltration furrows were constructed. This study focuses on two adjacent micro-basins, catchment Quillay and catchment Testigo. In figure 2.4 Quillay is shown on the left and Testigo on the right. In both catchments a limnigraph was installed to measure runoff. 6

13 Also a pluviometer was provided in watershed Quillay to measure the amount of rainfall on a daily basis (CONAF and JICA, 1998). Figure 2.4: Catchments Quillay and Testigo (After CONAF and JICA, 1998) In this study the headwater of catchments Quillay and Testigo were studied, as shown in figure 2.4. The source region of catchment Quillay is 2.88 hectares and Testigo is 3.60 hectares and their elevation is shown in figure 2.5 (CONAF and JICA, 1998). Figure 2.5: Elevation (meters) of the headwater of catchments Quillay and Testigo Since Testigo has no water harvesting techniques implemented, a comparative study can be performed between the two catchments. The main subject of this thesis was to investigate the effect of infiltration trenches on the discharge response of the watershed, in terms of increased water retention. 7

14 Chapter 3 Literature review 3.1 Land degradation The word degradation, from its Latin derivation, implies reduction to a lower rank. This rank is in relation to actual or possible uses. A reduction implies a problem for those who use the land. The productivity of the land declines when land becomes degraded unless steps are taken to restore that productivity (Blaikie and Brookfield, 1987). Land degradation consists of the deterioration of soil quality and vegetation. Soil degradation more specifically, refers to unfavorable alterations (physical, chemical or biological) of the soil properties. Soil degradation will reduce the productive capability of the soil, which completely impedes plant functions. Land degradation starts with the disappearance of the natural vegetation that covers the soil or with the broken up soil. The land is therefore subject to direct solar radiation and excessive oxygenation. This causes death to the living organisms and an acceleration of the biodegradation of the humus which causes aggregates to disappear and porosity to decrease. As a result, water and air do not easily circulate, the surface gets compacted and can even become impermeable. Water can therefore run off the soil instead of becoming stored (Gualterio, 2006). 3.2 Surface runoff The surface runoff process When rain falls, the first drops of water are intercepted by the leaves and stems of the vegetation. This is called interception storage. As the rain continues, water reaching the soil surface infiltrates into the soil until the rate of rainfall (intensity) exceeds the infiltration capacity of the soil. Ditches and other depressions are than filled. Thereafter runoff is generated (Critchley et al., 1991). The infiltration capacity depends on the texture of the soil, its structure as well as on the antecedent soil moisture content (previous rainfall). The initial infiltration capacity is high, so when it rains there will be a part of the rainfall that is withheld. But as rainfall continues, the infiltration capacity decreases as well as retention. Therefore the amount of runoff is increased. Finally, the infiltration capacity reaches a steady state value. The process of runoff generation continues as long as the rainfall intensity exceeds the actual infiltration capacity but stops when the rate of rainfall drops below the actual rate of infiltration. The relationship between rainfall, infiltration and runoff is given in figure 3.1 (Critchley et al., 1991). 8

15 Figure 3.1: Relationship between precipitation, infiltration and discharge (inch hr -1 ) (Critchley et al., 1991) Factors affecting runoff and erosion Surface runoff causes erosion (physical movement of soil) and this erosion is often of great magnitude, irreversible and may in extreme cases lead to total loss of soil. Erosion is therefore the one process of land degradation that has the main impact. Consequently, land degradation causes a disruption of the natural soil balance, increases runoff and erosion (intensification) and causes a loss in productivity. According to the Agricultural and Livestock Service, circa 60% of the forest and agricultural areas in Chile show moderate to very severe erosion (figure 3.2). 7% 26% 30% 37% Severe Moderate Mild Unaffected Figure 3.2: Desertification of Chile (After Soto, 1999) There are a number of factors that influence runoff and hence erosion (see figure 3.3) (Critchley et al., 1991): Antecedent moisture content Time to ponding will be shorter if the soil has a high antecedent moisture content. The topographic condition Investigations on experimental runoff plots have shown that steep slope plots yield more runoff than those with gentle slopes. Precipitation Rainfall characteristics such as frequency, intensity and duration of the precipitation have a direct influence on the occurrence and volume of runoff. Original material of the soil The infiltration capacity depends on the porosity of the soil which determines the water storage capacity and affects the resistance of water to flow into deeper layers. The porosity differs from one soil type to the other and is the highest for loose, sandy soils. 9

16 Vegetation coverage The amount of rain lost to interception storage depends on the kind of vegetation and its growth stage. Dense vegetation shields the soil from the raindrop impact. Therefore deforestation leads to more erosion on because the impact of energy from raindrops on the soil surface is increased. Therefore projects such as Erosion control and afforestation in watersheds in the semi-arid region of Chile are important. In addition, the root system increases the soil porosity and therefore allows more water to infiltrate. Vegetation also retards the surface flow, giving the water more time to infiltrate and to evaporate, and it results in higher soil organic carbon contents, nts, seriously improving stability of soil aggregates and promoting porosity. Precipitation Soil type Erosion Topography Vegetative cover Figure 3.3: Factors affecting soil erosion (after Gualterio, 2006) The most important erosion agents are wind and water, but water erosion is the most significant one in Chile. The most important problems with erosion are found in the altiplano sector, the foothills and mountains of the Andes. Wind erosion is most pronounced in the steppes of Patagonia, where the soil remains dry during intense spring and summer winds that easily remove and transport fine soil particles (Gualterio, 2006). Water erosion begins with the impact of a raindrop on the soil surface which disperses the finest soil particles in many different ways and loosens the particles from the soil aggregates. These particles are carried along by rainfall runoff. Precipitation often comes in the form of short bursts of high intensity rainfall. The soil capacity of infiltration is surpassed due to these high rain intensities ies and consequently, sheet erosion begins. When runoff becomes organized in little furrows (figure 3.4), the speed and kinetic energy will increase (Gualterio, 2006). Figure 3.4: Formation of furrows in catchment Testigo in Alto Loica 10

17 When speed and kinetic energy increase, landslides in the form of plates or displacements over a short distance without rupture of the surface will accompany erosion, forming little ridges perpendicular to the slope (Gualterio, 2006). Finally, these can develop into gullies with a complete loss of the soil in the affected sector (see figure 3.5). Figure 3.5: Ditch formation in catchment Testigo in Alto Loica 3.3 Water harvesting techniques Erosion leads to land degradation resulting in less water infiltrating in the soil and less water available for plant production. While irrigation may be the most obvious response to drought, it has proved costly. It also relies on advanced technology, which is lacking in most semi-arid areas. There is now increasing interest in a low cost alternative, namely water harvesting. Collecting or harvesting precipitation, often called water harvesting is the general name used for all the different techniques to collect and store runoff. By doing so the infiltration in the soil is increased and water is once again available for plant production. Water harvesting techniques can also act as an erosion control measure to reduce land degradation hazards, because instead of runoff being left to cause erosion, it is harvested and utilized (Critchley et al., 1991). The principle of water harvesting is given in figure 3.6. Catchment Storage runoff Cultivated area Figure 3.6: The principle of water harvesting 11

18 In general all water harvesting systems must have the following three components (Oweis and Hachum, 2004): Catchment area The area where the runoff is produced is called the catchment area. It is the part of the land that contributes some or all its share of rainwater to another area. The amount of water that can be harvested depends on the runoff efficiency in the catchment area. The area can be as small as a few square meters or as large as several square kilometers. Storage The storage facility is the place where runoff rainwater is held from the time it is collected until it is used. Storage can be in reservoirs or in the soil profile as soil moisture and in groundwater aquifers. Target area The harvested water can finally be used in a target area such as a cultivated area. Water harvesting can be considered as a rudimentary form of irrigation. The difference is that with water harvesting the farmer has no control over timing. Water can only be harvested when it rains. However, the available rain can be concentrated on a smaller area, so reasonable yield will still be achieved. Of course in a year of severe drought there may be no runoff to collect, but an efficient water harvesting system will improve plant growth in the majority of years (Critchley et al., 1991). 3.4 Classification of water harvesting techniques Water harvesting techniques can collect rainfall runoff for different uses, by linking a runoffproducing area with a separate runoff-receiving area (figure 3.7). Rainwater-harvesting systems (RWH) are typically classified into three categories based on the size of the runoff-producing area, namely in situ RWH, micro-catchment system and macro-catchment system (Mbilinyu et al., 2005) In situ RWH Figure 3.7: Components of a RWH system (Young et al., 2001) The first category of rainwater-harvest systems is in situ RWH or on-farm systems. This means rainfall is captured where it falls. It is basically the prevention of net runoff from a given cropped area by retaining rainwater and prolonging the time for infiltration (Mbilinyu et al., 2005). 12

19 3.4.2 Micro-catchment system The micro-catchment system is a second category of RWH system. It involves a distinct division between a runoff-generating catchment area (CA) and a cultivated basin (CB) where the runoff is stored in the root zone and productively used by plants. This principle is given in figure 3.7. The CA and CB are adjacent to each other in a micro-catchment system (Mbilinyu et al., 2005). The catchment length is usually between 1 and 30 meters and the ratio between CA and CB is normally between 1:1 and 1:3 (Critchley et al., 1991). The farmer has control, within his farm, over both the catchment and the target area. An example is shown in figure 3.8. This is an open-ended structure in V shape. Runoff is collected and stored in the infiltration pit. Figure 3.8: V -shaped micro-catchments (Critchley et al., 1991) Micro-catchment systems provide many advantages over other irrigation techniques (Oweis and Hachum, 2004): It is simple to construct. All the components are constructed within farm boundaries. This is an advantage from the point of view of maintenance and management. It can be built rapidly using local materials and manpower. Runoff water does not have to be transported or pumped, so it is relatively inexpensive. However, the principle of micro-catchment systems causes loss of productive land. Therefore it is only practiced in the drier environments where cropping is most risky and farmers are willing to allocate a part of their farm to be used as a catchment (Oweis and Hachum, 2004) Macro-catchment system The third category is macro-catchment system or an external catchment system, which is characterized by having runoff water collected from relatively large CA s. The catchment is usually 30 to 200 meters long and the ratio between CA and CB is usually between 2:1 and 10:1 (Critchley et al., 1991). The catchment area is located outside the cropped area, where individual farmers have little or no control over it (Mbilinyu et al., 2005). An example of a macro-catchment system is given in figure 3.9. Trapezoidal bunds are used to enclose larger areas (up to 1 ha) and to impound larger quantities of runoff. The name is derived from the structure (Critchley et al., 1991). 13

20 Figure 3.9: Trapezoidal bunds (Critchley et al., 1991) The larger the size of the catchment the larger the distance runoff water has to surpass and the lower the percentage of runoff collected. So the smaller individual catchments (micro-catchments) have higher runoff efficiency (volume of runoff per unit of area). This is another advantage of microcatchments (Critchley et al., 1991). 3.5 The infiltration furrow The infiltration furrow system is a micro-catchment system that is generally used for soil and water retention in Chile. The main objectives of the use of the furrows are (CONAF and JICA, 1998b): Soil recuperation by catching the runoff water on slopes and thus reducing soil losses from hillslopes. Increasing the infiltration of runoff water into the soil. Reduction of superficial runoff and its speed. It is clear that infiltration furrows are a good tool to reduce land degradation hazards and because more water infiltrates, there is more water available for plant production. This will increase the survival chances of newly introduced plants. The infiltration trench is a non-sloping channel that is dug out in a slope. This is done perpendicular to the direction of the slope and parallel to the contour lines. In figure 3.10a an overview of the implementation phase of infiltration trenches in catchment Quillay is shown. An example of an infiltration furrow holding water is shown in figure 3.10b. The following specifications of the infiltration furrow are recommenced by CONAF and JICA (CONAF and JICA, 1998b): A width at the top between 0.52 to 1 meter A width at the base of 0.2 meter A depth between 0.2 and 0.4 meter A length between 2.5 and 5 meter 14

21 (a) (b) Figure 3.10: The infiltration furrow: a) Implementation of the trenches (CONAF and JICA, 1998b), b) Example in catchment Quillay The horizontal space between the infiltration trenches depends on the characteristics of the slope (from eight meters in moderate slopes to three meters in moderate to steep slopes). The construction cost of infiltration trenches can vary greatly depending on the configuration, location, site-specific conditions, etc. The construction cost in catchment Quillay is Chilean pesos for 100 lineal meters per hectare or euro for 100 lineal meters per hectare where 24 lineal meters can be constructed each day (CONAF and JICA, 1999). In catchment Quillay the infiltration trenches were implemented with the dimensions that are given in figure Figure 3.11: Dimensions of the furrows implemented in Quillay (After Tokugawa and Vargas, 1996) 15

22 The furrows in Quillay were not implemented with a standard interval. The location of the trenches in catchment Quillay is shown in figure 3.12, which was established in this study by taking following actions: 1. Measurement of the length and width of the trenches and the distance between each other 2. Marking the corners of the trenches with GPS 3. Protract the trenches on graph paper 4. Digitizing the trenches using ArcGIS software (ESRI, Redlands, CA) 5. Kriging the field data of catchment Quillay obtained with the digital elevation model 6. Adding the trenches on the results of the kriging process Figure 3.12: Location of the infiltration trenches in catchment Quillay. The elevation (meters) of the catchment surface is also shown. Implementation of the infiltration furrow in arid and semi-arid zones of Chile has increased strongly over the last years, from 52 hectares in 2001 to 2200 hectares in 2003, due to strong incentives (Pizarro et al., 2004). Water harvesting techniques have evolved and were developed centuries ago and although various water harvesting techniques are realized, few efforts are made to quantify the effect of these techniques. Therefore, this thesis investigates the effect of infiltration trenches on the reduction in runoff of bare slopes in Chile and discharge of the ephemeral channel. A combination of detailed field measurements and modeling with the HydroGeoSphere software package was used to visualize the effect of the infiltration trenches. This will be explained in the next chapter. 16

23 Chapter 4 Materials and methods In the first section the HydroGeoSphere software will be explained which was used to visualize the effect of the infiltration trenches. Before using it for evaluating this effect, the model first needed to be calibrated and to that end PEST was used. How this was done will be explained in the second section. The evaluation of the effect of infiltration trenches will not only be performed with HydroGeoSphere, but also by comparing soil physical properties in both catchments. This is done via field measurements and statistical analysis of the corresponding results. The latter is explained in the third and fourth sections. 4.1 HydroGeoSphere Models for water transport at the surface and subsurface are helpful tools for understanding physical processes and for validating scientific hypotheses. Although the 'blueprint' for physically-based, surface-subsurface models was first proposed over three and a half decades ago (Freeze and Harlan, 1969), its use has only become widespread in the past 15 years with the advent of inexpensive and powerful computers (Sudicky et al., 2008). Surface and subsurface water flow models can be differentiated into three classes: the uncoupled, the iterative coupled and the fully coupled models. Examples of fully coupled models are InHM (Vanderkwaak, 1999), ParFlow (Kollet and Maxwell, 2006) and HydroGeoSphere (Therrien et al., 2010). These models take into account all components of the hydrologic cycle and the governing equations for the surface and subsurface domains are simultaneously solved. This is in strong contrast to the previous generation of models, in which the equations were solved separately for each domain and were eventually assembled via iteration (Sciuto and Diekkrüger, 2010). The model used in this study is the integrated surface-subsurface, three dimensional, finite element model HydroGeoSphere. It is a powerful numerical simulator specifically developed for supporting water resource and engineering projects pertaining to hydrologic systems with surface and subsurface flow and mass transport components (Therrien et al., 2010). The HydroGeoSphere model has been successfully applied at a large range of scales, from relatively small scale subcatchments, e.g. Sudicky et al. (2008), to larger watersheds, e.g. Li et al. (2008). As the scale increases the computation time will increase. The HydroGeoSphere has definitely got the potential for small and large scale hydrological studies and will increase even more in the future, because the computing power of computers is getting stronger and these computers are also becoming more affordable. In the following subsections a short overview of the main processes of the HydroGeoSphere software used in this thesis are discussed. More detailed information can be found in Therrien et al. (2010). Also a pre-processor called Grid builder was used to generate the plots of Testigo and Quillay. Finally the necessary input and output files are handled. This is discussed in the last subsection. 17

24 4.1.1 Main processes There are three fluxes occurring: subsurface, surface and interface flux. Because the implementation of these fluxes is based on different equations, each flux will be discussed separately in the following subsections. In the last subsection flow boundary conditions are discussed Subsurface flow Flow transport in the porous medium is simulated in three dimensions using the Richards equation for variably saturated subsurface flow (Therrien et al., 2010): + ± = (4.1) where [ ] is the volumetric fraction of the total porosity occupied by the porous medium and is always equal to 1 except when a second porous continuum is considered for a simulation. The vector is the fluid flux [L T -1 ] and is given by: = ψ + z (4.2) where = ( represents the relative permeability of the medium [dimensionless] with respect to the degree of water saturation [dimensionless]. Water saturation is related to the water content [dimensionless] according to =. Van Genuchten (1980) proposed the following saturation-pressure relation: = ψ ψ < 0 = 1 ψ 0 (4.3) with the relative permeability given by: = 1 1 (4.4) where = 1, > 1 (4.5) and where is the pore-connectivity parameter and is equal to 0.5 for most soils, α [L -1 ] and β [dimensionless] are van Genuchten parameters, S e is an effective saturation [dimensionless]: = (4.6) where is the residual water saturation [dimensionless] and is related to the water content according to =. ψ [L] in equation 4.2 is the pressure head and Z [L] is the elevation head. The hydraulic conductivity tensor [L T -1 ] is given by: = (4.7) where g is the gravitational acceleration [L T -2 ], µ is the viscosity of water [M L -1 T -1 ], is the permeability tensor of the porous medium [L²] and is the density of water [M L -3 ]. in equation 4.1 represents the volumetric fluid exchange rate [L 3 L -3 T -1 ] between the subsurface domain and all 18

25 other types of domains supported by the model. It is positive for flow into the porous medium. Fluid exchange with the outside of the simulation domain, as specified from boundary conditions, is represented by Q [L 3 L -3 T -1 ]. This is a volumetric fluid flux per unit volume representing a source (positive) or a sink (negative) to the porous medium system. [dimensionless] is the saturated water content and is assumed equal to the porosity n (Therrien et al., 2010). The storage term forming the right-hand side of equation 4.1 is expanded in a way similar to that presented by Cooley (1971) and Neuman (1973) to describe subsurface flow in the saturated zone. They relate a change in storage in the saturated zone to a change in fluid pressure through compressibility terms and they also assume that the bulk compressibility of the medium is constant for saturated and nearly-saturated conditions. For unsaturated conditions it is assumed that the compressibility effects on storage of water are negligible compared to effects of changes in saturation. The storage term is expressed as follows: + (4.8) where is the specific storage coefficient [L -1 ] (Therrien et al., 2010) Surface flow The overland flow is simulated using the two-dimensional Saint Venant equations, which consist of three equations and are given by the following mass balance equation (Therrien et al., 2010): ф + + coupled with the momentum equation for the x-direction: + ± = = 0 0 (4.9) (4.10) and the momentum equation for the y-direction: = 0 0 (4.11) where ф is a surface flow domain porosity that integrates flows over uneven surfaces, h is the water surface elevation [L] and is the sum of the land surface elevation z 0 [L] and the depth of flow [L], and are the vertically averaged flow velocities in the x and y directions [L T -1 ] and is a volumetric flow rate per unit area representing external sources and sinks [L T -1 ]. The variables,, and used in equations 4.10 and 4.11 are dimensionless bed and friction slopes in the x and y directions. The friction slopes can be determined with the Manning, the Chezy or the Darcy-Weisbach equations. Only the Manning equations will be explained as it was used in this study. Information about the Chezy and the Darcy-Weisbach equations can be found in Therrien et al. (2010). Using the Manning equation the friction slopes are determined by: = (4.12) = 19 (4.13)

26 where is the vertically averaged velocity [L T -1 ] along the direction of maximum slope s, n x and n y are the Manning roughness coefficients [L -1/3 T] in the x and y directions. The momentum equations 4.10 and 4.11 can be simplified using equations 4.12 and 4.13 and this results in the following equations: = = where and are surface conductances [L T -1 ] and are given for the Manning equations by: (4.14) (4.15) = = [ ] [ ] (4.16) (4.17) The diffusion-wave equation which the HydroGeoSphere software uses to simulate surface flows is finally obtained by substituting equations 4.14 and 4.15 into the mass balance equation 4.9, which give the following simplified equation: ф 0 h 0 h 0 h ± = 0 Equation 4.13 can also be written in vectorial notation: where is the fluid flux [L T -1 ] and is given by: 0 ± 0 = ф = d + z where is a factor that accounts for the reduction in horizontal conductance from obstruction storage exclusion [dimensionless] (Therrien et al., 2010) Flow coupling The subsurface and surface are complex systems that behave in a coupled manner. Therefore fully coupled models such as HydroGeoSphere have a big advantage, namely a full coupling between the surface and subsurface flow can be accomplished. The approach, used to define the water exchange term between the two domains, uses a Darcy flux relation to transfer water from one domain to the other. The flux is computed from the difference in hydraulic head between the two domains. It assumes that the domains are separated by a (nonexistent) thin layer of porous material across which water exchange will occur. The water exchange term is given by (Therrien et al., 2010): = h h where a positive [T -1 ] represents flow from the subsurface to the surface as determined by equation 4.9, h is the subsurface water head [L] and h is the surface water head [L], [dimensionlesss] is the relative permeability for the exchange flux, is the vertical saturated hydraulic conductivity of the underlying medium [L T -1 ] and h is the coupling length [L] (Therrien et al., 2010). A low value of the coupling length coefficient represents a rapid exchange of flow between the two domains and a low leakance factor or coupling length requires more computation time (Sciuto and Diekkrüger, 2010). 20 (4.18) (4.19) (4.20) (4.21)

27 Flow boundary conditions In this study the boundary conditions were expressed in terms of flux (Neumann type). In particular, rainfall and evapotranspiration were applied to the surface and no boundary conditions were imposed for the bottom and lateral domains. Evapotranspiration is an important component of the water balance and for this particular reason it is discussed here. Actual evapotranspiration is modeled as a combination of plant transpiration and of evaporation and affects both subsurface and surface flow domains. Transpiration occurs within the root zone of the subsurface. The rate of transpiration is estimated using the following relationship (Therrien et al., 2010): = (4.22) where is a function of leaf area index [dimensionless], is a function of nodal water content [dimensionless], RDF is the time varying root distribution function, is the potential or reference evapotranspiration and is the canopy evapotranspiration that corresponds to the evaporation of water intercepted by the canopy. The vegetation term is expressed as equation 4.23, the root zone term is defined by equation 4.24 and the moisture content is expressed by 4.25: where: = 0, [1, ] = 0 0 = 1 0 = 1 = 1 (4.23) (4.24) (4.25) (4.26) (4.27) and where C1, C2 and C3 are fitting parameter [dimensionless], [L] is the effective root depth, z [L] is the depth coordinate from the soil surface, is the root extraction function [L 3 T -1 ], is the moisture content at the wilting point, is the moisture content at field capacity, is the moisture content at the oxic limit and is the moisture content at the anoxic limit. Below the wilting point moisture content transpiration is zero. Above that the transpiration increases to a maximum at the field capacity moisture content. This maximum is maintained up to the oxic moisture content, beyond which the transpiration decreases to zero at the anoxic moisture content. The roots will become inactive due to lack of aeration when the available moisture is larger than the anoxic moisture content (Therrien et al., 2010). Evaporation from the soil surface and subsurface soil layers is expressed as follows if we assume that evaporation occurs along with transpiration: = [1 ] (4.28) 21

28 where EDF is the evaporation distribution function that distributes the water extracted from the evaporative zone along the evaporative depth L e [L] and α* is a wetness factor [dimensionless] and is expressed as follow: (4.29) Where θ e1 is the moisture content above which full evaporation can occur and θ e2 is the limiting moisture content below which evaporation is zero. The interception storage capacity [L] represents the maximum quantity of water that can be intercepted by the canopy. It depends on LAI and the canopy storage parameter C int [L] and is given by (Therrien et al., 2010): (4.30) Grid builder The first step in applying HydroGeoSphere is grid generation for which Grid builder (McLaren, 2007) was used for designing the grids of catchments Testigo and Quillay. The finite element grid is generated automatically in a two-dimensional plane and is projected in the vertical to form a three- dimensional grid (McLaren, 2007). This projection is done during the HydroGeoSphere pre-processing phase. Although no general rules can be given about the optimal grid size, it is recommended to use the finest grid balanced with the availability of data and the required computational time to guarantee an accurate solution (Sciuto and Diekkrüger, 2010). A digital elevation model was used to define with Grid builder a two-dimensional, triangular-element mesh representing the top of both catchments. In figure 4.1 the two dimensional grid of catchment Testigo constructed with Grid builder is given. The grid is refined near the river. Figure 4.1: : Two dimensional grid of Testigo with a refining near the river 22

29 The grid of Quillay is given in figure 4.2 and is refined near the infiltration furrows. The infiltration trenches were incised into the surface mesh by lowering the nodes 0.20 m. The grid of Quillay is a much finer grid than Testigo. This higher refinement was done to obtain a good resolution, as shown in the right part of figure 4.2 Figure 4.2: Two dimensional grid of Quillay with a refining near river and infiltration furrows On these grids the field elevation data obtained with the digital elevation model was added and a kriging interpolation process started. The results of these interpolations can be visualized with external visualization software like Tecplot 360 and is given in figure 4.3 for catchment Testigo and in figure 4.4 for catchment Quillay. The grids in figure 4.3 and 4.4 are the finest grids. In order to compare simulations on these fine grids with simulations on coarse grids, a rectangular grid was made with an element length of 25 m for Testigo and Quillay. The coarse grid of Testigo is given in figure 4.5 and the coarse grid of Quillay is given in figure 4.6. Figure 4.3: Tecplot 3D-visualization of the surface elevation (meters) of Testigo after kriging. 23

30 Figure 4.4: Tecplot 3D-visualization of the surface elevation (meters) of Quillay after kriging Figure 4.5: Coarse grid of Testigo and tecplot 3D-visualization of the surface elevation (meters) Figure 4.6: Coarse grid of Quillay and tecplot 3D-visualization of the surface elevation (meters) 24

31 4.1.3 Input and output files There are four steps involved in solving a given problem using HydroGeoSphere: 1. Build the necessary input files for the pre-processor GROK 2. Run GROK to generate input data files for the actual processor HGS 3. Run HGS to solve the problem and generate output data files 4. Run the post-processor HSPLOT which generates output files suitable to be opened in Tecplot Input files All inputs are text-based files which can be written with every text editor such as notepad++, but they are to be saved with a specific extension. The main input file is the grok-file (extension: *.grok). This is a controlling file with a well-structured list of commands and data, specifying the conditions that have to be simulated. All the different possible commands are listed in the HydroGeoSphere manual (Therrien et al., 2010). An example of used grok-files for catchments Testigo and Quillay are given in appendix A. It concerns a simulation for the rainfall event described in chapter 5 ( 5.3.2), namely the simulations which are based on the finest grids and with evapotranspiration data. Among other things, precipitation and runoff data are added in the grok-file. Rainfall data was obtained through the cooperation between CONAF and JICA. A pluviometer (see figure 4.7) and an automated pluviometer with a tipping bucket system which was connected to a battery driven pluviograph was used to measure the amount of precipitation. Trap-like ticks were recorded whenever the bucket changed sides, once for every millimeter rain. The ticks had to be counted and digitalized manually. (a) (b) Figure 4.7: The pluviometer used in catchment Quillay: a) Exterior of the pluviometer, b) Collection of rainwater in a bottle Next to precipitation data, runoff data is needed. This data was also obtained through the cooperation between CONAF and JICA where runoff data was collected for both catchments Testigo and Quillay. The runoff measurements were determined using a limnigraph (figure 4.8a). This is based upon a V-shaped weir by constant measurements of the water level of the water passing the 25

32 triangle. A water level recorder (model: W-351, Yokogawa Weathac Corporation) was used to measure the water height. An example of a measuring paper is shown in figure 4.8b, whereby a continuous full line is drawn reflecting the height difference for every millimeter. The red line gives the differences in height in mm and the green line in cm. Measurements of the water level were performed from 1993 until (a) (b) Figure 4.8: Measuring the runoff: a) Limnigraph in catchment Quillay, b) Discharge observation paper (after Geeroms, 2009) The discharge was determined for both watersheds from the continuous water level measurements using the following equation (Tokugawa and Vargas, 1996): = 2 h (4.31) where Q [m³ s -1 ] is the discharge, the coefficient of discharge, [ ] the bottom angle of the V- shaped weir, g [9.81 m s -2 ] the gravitational acceleration and h [m] the water level. The grok-file can also refer to other input files, such as the mprops-, oprops- and etprops-files. Also the grid-files discussed in subsection can be used as input file. The mprops-file stands for material properties file (extension: *.mprops) and contains the data related to the porous subsurface part of the model. A few examples are hydraulic conductivity, porosity, specific storage The oprops-file stands for overland properties file (extension: *.oprops) and contains overland or surface properties such as the x friction, y friction and coupling length. Note that this is actually not a surface nor a subsurface parameter. The used values of the parameters in the mprops and oprops files in this study are discussed later in The etprops-file stands for evapotranspiration properties file (extension: *.etprops) and contains the data related to evapotranspiration. For Testigo data was used of Acacia caven and grass. For Quillay data was used of Acacia caven, grass and Eucalyptus globulus subsp. globulus. Using a GPS, the trees had been marked and this is shown in figure 4.9. The top area in this figure is catchment Testigo and the area underneath is catchment Quillay. 26

33 Figure 4.9: Trees in catchments Testigo (top) and Quillay (bottom) The introduction of evapotranspiration in the model is done by assigning parts of the top layer of the grid as evaporating elements. This is done via Grid builder using the data obtained from the GPS (figure 4.9). The top elements were therefore subdivided. Evapotranspiration parameters of Eucalyptus globulus subsp. globulus were assigned to the areas containing big trees (Acacia dealbata, Cupressus arizonica, Eucalyptus camaldulensis, Eucalyptus globulus, Maytenus boaria, Pinus and 27