List of Figures
List of Tables
2.0 STUDY AREA
3.2 Soil Moisture Characteristics
3.3 Soil Moisture Retention Curves
3.3.1 Pressure Plate Apparatus
3.4 Saturated Hydraulic Conductivity
3.4.1 Guelph Permeameter
3.5 van Genuchten Parameters
4.0 DESCRIPTION OF SWIM MODEL
4.2 Water Movement
4.2.1 Richards’ Equation
4.2.2 Hydraulic Properties
4.2.3 Initial and Boundary Conditions
4.3 Solute Transport
4.3.1 Advection-Dispersion Equation
4.3.2 Solute Initial and Boundary Conditions
4.4 Limitations of the Model
5.0 ANALYSIS AND RESULTS
5.2 Soil Moisture Characteristics
5.3 Model Conceptualization
5.4 Simulation of Water Balance Components
5.5 Concluding Remarks
In many arid and semi-arid regions, surface water resources are limited and ground water is the major source for agricultural, industrial and domestic water supplies. Because of lowering of water tables and the consequently increased energy costs for pumping, it is recognized that ground water extraction should balance ground water recharge in areas with scarce fresh water supplies. This objective can be achieved either by restricting ground water use to the water volume which becomes available through the process of natural recharge or by recharging the aquifer artificially with surface water. Both options require knowledge of the ground water recharge process through the unsaturated zone from the land surface to the regional water table.
This report entitled “Simulation of Soil Moisture Movement in a Hard Rock Watershed using SWIM Model” is a part of the research activities of ‘Hard Rock Regional Centre’ of National Institute of Hydrology, Roorkee, India. The purpose of this study is to simulate the soil moisture movement in a hard rock watershed through a numerical model and determine the ground water recharge from rainfall. The study has been carried out by Mr. C. P. Kumar, Scientist ‘F’ and Dr. B. K. Purandara, Scientist ‘E’.
LIST OF FIGURES
1. Drainage System of Barchi Watershed
2. Monthly Rainfall and Evaporation in Barchi Watershed during the Year 1996-97
3. Monthly Rainfall and Evaporation in Barchi Watershed during the Year 1997-98
4. Monthly Rainfall and Evaporation in Barchi Watershed during the Year 1998-99
5. Monthly Rainfall and Evaporation in Barchi Watershed during the Year 1999-2000
6. Guelph Permeameter
7. Components of the Soil Water and Solute Balances addressed by SWIM v2
8. Soil Moisture Retention Curve at Location 1 for Upper Soil Layer
9. Soil Moisture Retention Curve at Location 2 for Upper Soil Layer
10. Soil Moisture Retention Curve at Location 3 for Upper Soil Layer
11. Soil Moisture Retention Curve at Location 4 for Upper Soil Layer
12. Soil Moisture Retention Curve at Location 5 for Upper Soil Layer
13. Soil Moisture Retention Curve at Location 6 for Upper Soil Layer
14. Soil Moisture Retention Curve at Location 7 for Upper Soil Layer
15. Soil Moisture Retention Curve at Location 8 for Upper Soil Layer
16. Soil Moisture Retention Curve at Location 1 for Lower Soil Layer
17. Soil Moisture Retention Curve at Location 2 for Lower Soil Layer
18. Soil Moisture Retention Curve at Location 3 for Lower Soil Layer
19. Soil Moisture Retention Curve at Location 4 for Lower Soil Layer
20. Soil Moisture Retention Curve at Location 5 for Lower Soil Layer
21. Soil Moisture Retention Curve at Location 6 for Lower Soil Layer
22. Soil Moisture Retention Curve at Location 7 for Lower Soil Layer
23. Soil Moisture Retention Curve at Location 8 for Lower Soil Layer
LIST OF TABLES
1. Soil Moisture Retention Data for Upper Soil Layer
2. Soil Moisture Retention Data for Lower Soil Layer
3. van Genuchten Parameters for Upper Soil Layer
4. van Genuchten Parameters for Lower Soil Layer
5. Hydraulic Property Input for Upper Soil Layer
6. Hydraulic Property Input for Lower Soil Layer
7. Water Balance Components for the Barchi Watershed
A very large fraction of the water falling as rain on the land surfaces of the earth or applied irrigation water moves through unsaturated soil during the subsequent processes of infiltration, drainage, evaporation, and the absorption of soil-water by plant roots. The water movements in the unsaturated zone, together with the water holding capacity of this zone, are very important for the water demand of the vegetation, as well as for the recharge of the ground water storage. A fair description of the flow in the unsaturated zone is also crucial for predictions of the movement of pollutants into ground water aquifers.
A number of simulation models are available for investigating the soil water balance. SWIM (Soil Water Infiltration and Movement) is a physically based, isothermal, one dimensional model of water flow through the soil coupled with a simple crop water extraction model in which the growth of the canopy and of the root system is a predetermined input. SWIM is driven by rainfall and potential evaporation, and so appears to be more appropriate than few other similar models if the available meteorological data are limited.
The present study aims at modelling of soil moisture movement in Barchi watershed (Karnataka) using SWIM. Field and laboratory investigations were carried out to determine the saturated hydraulic conductivity at eight locations using Guelph Permeameter and soil moisture retention characteristics using the Pressure Plate Apparatus. The van Genuchten parameters of soil moisture retention function and hydraulic conductivity function were obtained through non-linear regression analysis. Daily rainfall and evaporation data of Barchi for the period 1996-97 to 1999-2000 were used for the simulations. Water balance components like runoff, evapotranspiration and drainage (groundwater recharge from rainfall) were determined through SWIM.
Most of the processes involving soil-water interactions in the field, and particularly the flow of water in the rooting zone of most crop plants, occur while the soil is in an unsaturated condition. Unsaturated flow processes are in general complicated and difficult to describe quantitatively, since they often entail changes in the state and content of soil water during flow. Such changes involve complex relations among the variable soil wetness, suction, and conductivity, whose inter-relations may be further complicated by hysteresis. The formulation and solution of unsaturated flow problems very often require the use of indirect methods of analysis, based on approximations or numerical techniques. For this reason, the development of rigorous theoretical and experimental methods for treating these problems was rather late in coming. In recent decades, however, unsaturated flow has become one of the most important and active topics of research and this research has resulted in significant theoretical and practical advances.
Subsurface formations containing water may be divided vertically into several horizontal zones according to how large a portion of the pore space is occupied by water. Essentially, we have a zone of saturation in which all the pores are completely filled with water, and an overlaying zone of aeration in which the pores contain both gases (mainly air and water vapour) and water. The latter zone is called the unsaturated zone. Sometimes the term soil water is used for the water in this zone.
The vertical movement of soil moisture in the liquid phase between the surface and the water table can be subdivided into the following three categories according to predominant forces involved.
Infiltration and exfiltration
Alternate wetting and drying of soil surface during consecutive storm and interstorm periods will cause a penetration of the medium by an unsteady wave like diffusion of liquid soil moisture into the soil during wet surface (storm) periods under the complementary effects of capillarity and gravity and out of the soil during dry surface (interstorm) periods when capillarity opposes gravity. With increasing depth of penetration, diffusion reduces the soil moisture gradients and thus reduces the effect of capillarity until moisture movement becomes dominated by gravity. The depth at which surface induced capillary forces become negligible determines the penetration depth of the surface process and is used to define the thickness of the zone of soil moisture. The presence of transpiring vegetation adds another mechanism for moisture extraction distributed over a depth which is related to root structure.
Liquid soil moisture moves out of the bottom of the zone of soil moisture and percolates downward under the domination of gravity forces until it encounters the increasing soil moisture gradients lying above the water table. At some depth upward capillary forces will be prominent defining the bottom of this intermediate zone.
Between the water table and the intermediate zone there is a capillary fringe in which gravity and capillarity again jointly govern the liquid soil moisture movement.
When water is supplied to the soil surface, whether by precipitation or irrigation, some of the arriving water penetrates the surface and is absorbed into the soil, while some may fail to penetrate but instead accrue at the surface or flow over it. The water which does penetrate is itself later partitioned between that amount which returns to the atmosphere by evapotranspiration and that which seeps downward, with some of the latter reemerging as stream flow while the remainder recharges the ground water reservoir.
For analytical studies on soil moisture regime, critical review and accurate assessment of the different controlling factors is necessary. The controlling factors of soil moisture may be classified under two main groups viz. climatic factors and soil factors. Climatic factors include precipitation data containing rainfall intensity, storm duration, interstorm period, temperature of soil surface, relative humidity, radiation, evaporation, and evapotranspiration. The soil factors include soil matric potential and water content relationship, hydraulic conductivity and water content relationship of the soil, saturated hydraulic conductivity, and effective medium porosity. Besides these factors, the information about depth to water table is also required.
The amount of water that may be extracted from an aquifer without causing depletion is primarily dependent upon the ground water recharge. Thus, a quantitative evaluation of spatial and temporal distribution of ground water recharge is a pre-requisite for operating ground water resources system in an optimal manner.
Rainfall is the principal means for replenishment of moisture in the soil water system and recharge to ground water. Moisture movement in the unsaturated zone is controlled by capillary pressure and hydraulic conductivity. The amount of moisture that will eventually reach the water table is defined as natural ground water recharge. The amount of this recharge depends upon the rate and duration of rainfall, the subsequent conditions at the upper boundary, the antecedent soil moisture conditions, the water table depth and the soil type.
The theory for transient isothermal flow of water into nonswelling unsaturated soil is well understood and has been developed to a large extent in terms of solutions of the non-linear Richards equation. In the field, the description of infiltration is highly complicated since the initial and boundary conditions are usually not constant while the soil characteristics may vary with time and space. In view of this, most efforts in recent past, have been concentrated on seeking numerical solutions.
The governing partial differential flow equation can be interpreted numerically by a finite difference, a finite element or a boundary element technique. Then a discretization scheme is applied for a system of nodal points that is superimposed on the soil depth-time region under consideration. Implementing the appropriate initial and boundary conditions then leads to a set of (linear) algebraic equations that can be solved by different methods. The operation by means of such a mathematical model is termed simulation, while the model is called simulation model.
The soil water movement may be modelled mathematically from bases provided by:
(a) the soil moisture characteristic,
(b) equations describing the volume flux of water and water vapour in response to potential gradients, and
(c) the law of continuity of matter and additionally, in the case of evaporation, the law of continuity of heat energy.
The objective of the present study is to simulate the movement of soil moisture in Barchi watershed (sub-basin of Kali river in North Kanara district of Karnataka) using the SWIM model. The study includes
- Measurement and determination of saturated hydraulic conductivity and soil moisture retention characteristics.
- Modelling of soil moisture movement using the SWIM model. Daily rainfall and evaporation data of Barchi for the period 1996-97 to 1999-2000 were used for the study.
- Determination of water balance components like runoff, evapotranspiration and drainage (recharge to groundwater from rainfall).
The SWIM (Soil Water Infiltration and Movement) is a software package developed by Division of Soils, CSIRO, Australia (Verburg et al., 1996) for simulating infiltration, evapotranspiration, and redistribution. The major features of the model include the ability to deal with:
- Layered and gradational soils such as occur in field soils where hydraulic properties vary with depth down the profile, either abruptly or gradually.
- Saturated/unsaturated conditions as can occur at layer interfaces, which result in locally perched water.
- Surface ponding as can occur under high rainfall intensities.
- Surface runoff, where ‘excess’ water can be removed from the system.
- Surface sealing, where the properties of the surface may vary directly as a function of rainfall energy, and hence as a function of time.
- Rainfall dynamics, so that real storm intensities (down to 1-minute resolution and below) can be simulated.
- Solute transport.
- Vapour flow, hysteresis, bypass flow, osmotic effects, and potential subsurface downslope flow.
- ‘Cultivations’ or ‘disturbances’ of the soil surface which enable the application of dry fertilizer (solute) and resetting of the surface conductance and surface roughness values at specified times.
SWIM has already been used world-wide in a variety of studies. It was originally written assuming that a preprocessor would be used to interface with the user. A preprocessor is not yet available; hence current input facilities (Version 2.1) are less than ideal.
2.0 STUDY AREA
The Barchi watershed upstream of Barchi is located in the leeward side of western ghat and is a sub-basin of Kali river. It lies in Haliyala taluk of Karwar (North Kanara) district in Karnataka. The location and drainage system of Barchi watershed is shown in Figure 1.
The Barchinala stream originates from Thavargatti in Belgaum district at an altitude of about 734 m, 20 km north of Dandeli and flows through North Kanara district of Karnataka State. The catchment is relatively short in width and river flows in a southerly direction and joins the main Barchi river near the gauging site. The geographical area covered by Barchi watershed is 21.126 km2. The watershed lies between 74o36’ and 74o39’ East longitudes, and 15o18’ and 15o24’ North latitudes.
High land region consists of dissection of high hills and ridges forming part of the foot hills of western ghats. It consists of steep hills and valleys intercepted with thick forest. The slopes of the ghats are covered with dense deciduous forest. Forest cover occupies around 80% of the study area. The watershed is mainly covered with Bamboo, Teak and mixed plantations. The brownish and fine-grained soils are the principal types of soils found in the area. The following land uses were observed at the locations of field studies (Figure 1):
1. Bamboo plantation (near gauging site)
2. Teak plantation (ridge)
3. Mixed forest, disturbed fire.
4. Mixed forest with bamboo
5. Soil profile with high litter content
6. Agricultural land
7. Mixed forest with high litter content
8. Bamboo and mixed forest
The stream gauging site is located at an elevation of 480 m, where the nala crosses Dandeli-Thavargatti road, about 5 km from Dandeli. The stream is a 4th order stream and joins main Barchi river downstream of the gauging site. A full fledged meteorological station, maintained by Water Resources Development Organisation (WRDO), Karnataka, is located near the gauging site.
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Figure 1: Drainage System of Barchi Watershed
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Figure 2: Monthly Rainfall and Evaporation in Barchi Watershed during the Year 1996-97
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Figure 3: Monthly Rainfall and Evaporation in Barchi Watershed during the Year 1997-98
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Figure 4: Monthly Rainfall and Evaporation in Barchi Watershed during the Year 1998-99
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Figure 5: Monthly Rainfall and Evaporation in Barchi Watershed during the Year 1999-2000
The Barchi raingauge station is located at 15o18’ N and 74o37’ E. Average annual rainfall for the watershed is 1500 mm, majority of which occurs during the south-west monsoon period. Figures 2 to 5 present the variation of monthly rainfall and evaporation in Barchi watershed during the years 1996-97 to 1999-2000 respectively.
Depth to water table varies between 4 to 12 metres during pre- and post-monsoon periods. The yield of borewells in the study area is found to vary between 120 gallons per hour to 1170 gallons per hour.
The present study involves modelling of soil moisture movement in Barchi watershed using the SWIM model. The following steps were undertaken for the study.
- Field investigations – measurement of saturated hydraulic conductivity at 8 locations using Guelph Permeameter and soil sampling.
- Laboratory investigations – Determination of saturated moisture content, and soil moisture retention characteristics using the Pressure Plate Apparatus.
- Modelling of soil moisture movement using the SWIM model. Daily rainfall and evaporation data of Barchi for the period 1996-97 to 1999-2000 were used for the study. Water balance components like runoff, evapotranspiration and drainage (recharge to groundwater from rainfall) were determined through SWIM.
Details of equipment and procedures adopted for field and laboratory investigations are presented below. Description of SWIM model is discussed in the next chapter.
3.2 Soil Moisture Characteristics
Quantitative measurements of soil physical properties are required for many purposes. In the area of land management, one may wish to know whether a particular management scheme will increase or decrease infiltration, runoff, erosion, leaching, salinization etc. We may need to predict material transport, such as the depth to a wetting front, position of a seepage face, time of arrival of a tracer plume, cumulative evaporation etc.
Any measurement of soil water in the field depends upon sampling at a given location, both in area and depth of soil profile, at a given time or times. These samples are then used to estimate the water condition of the entire area. Many methods are sufficiently accurate to measure the water condition in a given sample at a given time. Difficulty comes when one tries to apply these conditions to a large area or at a different time. In reality, the water condition measured is a transient one in a system that is continuously changing in three-dimensional space and time and the situation would likely be different at any other location at the same time, or at the same location at a different time.
In order to evaluate completely the condition of water in soil, one must know the energy of the water, the amount of water in the soil, and how these conditions change in space and time. This requires a complete understanding of water movement and flow in soils. Such complete evaluations of soil water conditions are not easily made, and are available only under controlled laboratory conditions.
There are two general reasons for measuring soil water. One is to determine the moisture content of a soil, that is, the amount of water contained in a unit mass or volume of soil. This information is necessary to calculate the water needed to restore the soil water in the root zone of the crop. The second reason is to determine the magnitude of the soil water potential, which is the negative of the work that must be done to remove a unit amount of the most loosely held water.
Plant response to water appears to be more closely related to the water potential than any other single factor, although the velocity of movement of water to the absorbing root is an important consideration. This movement rate is strongly related to the potential. Because of this relation, one desires to know the potential of the soil water whenever he is concerned about plant response. Knowledge of the soil water potential is also desired by irrigators since it indicates directly when water should be applied.
Prediction of infiltration is important in the design of irrigation areas and for the estimation of runoff in catchment management studies. Many predictive models exist and various methods have been employed in measuring infiltration behaviour. The proper evaluation of infiltration behaviour depends on knowledge of the hydrological soil properties.
Saturated hydraulic conductivity and unsaturated hydraulic conductivity are related to the degree of resistance from soil particles when water flows in pores. These resistances are affected by the forms, sizes, branchings, jointings, and tortuosities of pores as well as viscosity of water. In addition, unsaturated hydraulic conductivity is affected markedly by the volumetric water content of soil.
The relation between matric potential and volumetric water content in a soil is termed as the soil moisture characteristic curve because the curve is characteristic of each soil. The differences among soil moisture characteristic curves are attributed primarily to the differences in pore size distribution among soils. These curves are sensitive to the changes in bulk densities and disturbances of soil structures. In addition, the curves generally show hysteresis according to the wetting or drying of soils.
3.3 Soil Moisture Retention Curves
The graph giving the relation between soil moisture tension and soil moisture content is called moisture retention curve or soil moisture characteristic. If the tension is expressed as the logarithmic value of cm water, the graph is referred to as a pF-curve. Moisture retention curves are used:
- to determine an index of the available moisture in soil (the portion of water that can be readily absorbed by plant roots) and to classify soils accordingly, e.g. for irrigation purposes,
- to determine the drainable pore space (effective pore space, effective porosity, specific yield) for drainage design,
- to check changes in the structure of a soil, e.g. caused by tillage, mixing of soil layers etc.,
- to ascertain the relation between soil moisture tension and other physical properties of a soil (e.g. capillary conductivity, thermal conductivity, clay and organic matter content).
Clay soils show a slow and regular decrease in water content with increasing pF tension. Sandy soils may show only a slight decrease in moisture content in the lower pF range till the point where only a small rise in pF causes a considerable discharge of water due to a relatively large number of pores in a particular diameter range. The intersection point of the curve with the volumetric water content axis (tension: 1 cm water, pF = 0) gives the water content of the soil under nearly saturated conditions, which means that this point almost indicates the total pore space percentage (if no air entrapment has taken place). The zero moisture content is based on the oven-dry condition (105 oC), corresponding to a pF of approximately 7.
To construct the moisture retention curve of a soil sample, the moisture content of that sample must be measured. This is done by equilibrating the moist soil sample at a succession of known pF values and each time determining the amount of moisture that is retained. If the equilibrium moisture content (expressed preferably as volume percentage) is plotted against the corresponding tension (pF), the moisture retention curve (pF-curve) can be drawn. There is no single method of inducing the whole range of tensions from pF = - ¥ (total saturation) to pF = 7 (oven dry).
The ceramic plates equipment is suitable for determination of pF-curves in the pF range of 2.0-4.2 (0.1-15 bar of suction). Soil moisture is removed from the soil samples by raising air pressure in an extractor. A porous ceramic plate serves as a hydraulic link for water to move from the soil to the exterior of the extractor. The high-pressure air will not flow through the pores in the plate since the pores are filled with water. The smaller the pore size, the higher the pressure that can be exerted before air will pass through. During an experimental run, at any set pressure in the extractor, soil moisture will flow around each of the soil particles and out through the ceramic plate and outflow tube. Equilibrium is reached when water flow from the outflow tube ceases. At equilibrium, there is an exact relationship between the air pressure in the extractor and the soil suction (and hence the moisture content) in the samples. Accuracy of equilibrium values will be no more accurate than the regulation of air supply; therefore the pressure control panel has independent double regulators.
For each soil type, the characteristic pF-curve may be developed. These curves relate the soil suction to its moisture content. This relationship is important in studies of soil moisture movement and quantity and availability of soil moisture for plant growth.
3.3.1 Pressure Plate Apparatus
It consists of a ceramic pressure plate cell mounted in a pressure vessel, with the outflow tube running through the vessel wall to the atmosphere and soil sample held in place on the porous ceramic surface of the cell. Each ceramic pressure plate cell consists of a porous ceramic plate covered on one side by a thin neoprene diaphragm sealed to the edges of the ceramic plate. An internal screen between the plate and diaphragm provides a passage for flow of water. An outlet stem running through the plate connects this passage to an outflow tube fitting which connects to the atmosphere outside of the extractor.
To use the ceramic pressure plate cell, one or more soil samples are placed on the porous ceramic surface and held in place by retaining rings of appropriate height. The soil samples, together with the porous ceramic plate, are then saturated with water. This is usually done by allowing excess water to stand on the surface of the cell for several hours. When the saturation is complete, the cell can be mounted in the pressure vessel. Air pressure is used to effect extraction of moisture from the soil samples under controlled conditions.
As soon as air pressure inside the chamber is raised above the atmospheric pressure, higher pressure inside the chamber forces excess water through the microscopic pores in the ceramic plate and out through the outlet stem. The high pressure air, however, will not flow through the pores in the ceramic plate since the pores are filled with water and the surface tension of water, at the gas-liquid interface at each of the pores, supports the pressure similar to a flexible rubber diaphragm.
The maximum air pressure that any given wetted porous ceramic plate can stand before letting air pass through the pores, is determined by the diameter of pore. The smaller the pore sizes, the higher the pressure needed for air to pass through. The pressure value that finally breaks down the water meniscus, is called the “bubbling pressure” or the “air entry value” for the porous plate. Pressure plate cells must always be used at air pressure extraction values below the “bubbling pressure” or “air entry value” for the cell.
During an experimental run, for any set air pressure in the extractor, soil moisture will flow from around each of the soil particles and out through the ceramic plate until the effective curvature of water films throughout the soil are same as at the pores in the plate. When this occurs, an equilibrium is reached and the flow of moisture ceases. When air pressure in the extractor is increased, flow of soil moisture from the samples starts again and continues until a new equilibrium is reached. At equilibrium, there is an exact relationship between the air pressure in the extractor and the soil suction (and hence the moisture content) in the samples. For example, if air pressure in the extractor is maintained at 1/3 bar, the soil suction in the samples at equilibrium will be 1/3 bar. If air pressure is maintained at 1 bar, the soil suction at equilibrium will be 1 bar.