Estimation of Groundwater Recharge under various land ... - RMZ

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186 Purandara B. K., Venkatesh B. & V. K. Choubeyof all these maximum intensity valueswere computed using the Weibull plottingposition procedure. A multipleregression model is used to develop arelationship of intensity versus durationand frequency of the form of, I =112,47 D –0.341 T 0.21 , The multiple correlationcoefficient obtained was 0.80Field InvestigationsField experiments were carried out forthe determination of saturated hydraulicconductivity in three typical zonesby using Disc permeameter (Perroux& White, 1988) and Guelph permeameter.In each location a plot of 10 m/10m was selected and carried out 6–8 experimentsin order to get a proper representation.Data has been subjected tostatistical analysis to get log mean values.LSD and F tests were also carriedout for the analysis.Laboratory InvestigationsLaboratory investigations includeddetermination of saturated moisturecontent, and soil moisture retentioncharacteristics using the pressure plateapparatus.charge to groundwater from rainfall)were determined through SWIM.SWIM is an acronym that stands forSoil Water Infiltration and MovementModel. It is a software package developedwithin the CSIRO Division ofsoils for simulating infiltration, evapotranspiration,and redistribution. Themodel is based on a numerical solutionof the Richards’ equation and theadvection-dispersion equation. It canbe used to simulate runoff, infiltration,redistribution, solute transportand redistribution of solutes, plantuptake and transpiration, soil evaporation,deep drainage and leaching. Soilwater and solute transport properties,initial conditions, and time dependentboundary conditions (e.g., precipitation,evaporative demand, solute input)need to be supplied by the user inorder to run the model. The governingpartial differential equation (Richards’equation) applicable for one-dimensionalflow in the unsaturated zone canbe written as:ModelingDaily rainfall and evaporation data of1986 to 2000 were used for the study.Water balance components like runoff,evapotranspiration and drainage (re-where,θ = volumetric water content[cm 3 /cm 3 ]; water/soilt = time [h]x = distance into the soil [cm]K = hydraulic conductivityRMZ-M&G 2010, 57
Estimation of Groundwater Recharge under various land covers in parts of ...187[cm 2 /(cm/h); water/soil]= matric potential [cm]; waterz = gravitational potential[cm] andS = sink strength[cm 3 /(cm 3 /h)];water/soilThe model deals with a one-dimensionalsoil profile. For a vertical soil profile,this means that it may be vertically inhomogeneous,but must be horizontallyuniform. This assumption has two consequencesof importance in many commonsimulations. There is only one hydraulicconductivity function for eachlayer, so that any macropore, or bypassflow can only accounted for in a limitedway. Secondly, the calculated soluteconcentrations apply to the whole soillayer, which means that there is no concentrationgradient from the bulk soilto near the root surface. The presenceof such a concentration gradient mayin reality affect the soil osmotic potentialand hence water and solute uptake(Verburg et al, 1996).Model ConceptualizationIn order to simulate the water balancecomponents of the study area, a soilprofile in each zone, viz., coastal, upghat and plateau areas were consideredwith a thicknes of about 150 cm.Vapour conductivity is not taken intoconsideration nor is the effect of osmoticpotential. There are two hydraulicproperty sets (for upper and lowersoil horizons) that applied to 16 nodesof the 150 cm deep profile. Hysterisesis not taken into account. Initially,there is no water ponded on the surface.Runoff is governed by a simple powerlaw function and a surface conductancefunction. No by pass flow wasincluded. A matric potential gradient of0, i.e., `unit gradient’ has been appliedas bottom boundary condition throughout the simulation. Cumulative rainfalland evaporation records (daily) for theperiod 1986–2000 were given as theinput for determination of water balancecomponents (runoff, evapotranspirationand drainage). The model parameters(soil hydraulic properties andmoisture characteristics) were actuallymeasured in the field and laboratory.Therefore, the model does not requireany calibration as such.Input Data for SWIM Model1. Rainfall: Based upon the availableinformation, two distinct soil layerswere identified. The followinginput data was used for simulationof soil moisture movement throughSWIM. Daily rainfall data of Honnavar,Barchi and Siddapur wereused.2. Evaporation: Daily evaporationdata of Honnavar (coastal), Barchi(plateau) and Siddapur (Up ghat)were considered for the analysis.3. Saturated Hydraulic Conductivity:RMZ-M&G 2010, 57
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