Estimation of Groundwater Recharge Using WetSpass and MODFLOWPutika Ashfar K February 2016

IntroductionEvaluation of the recharge rate is a prerequiste for efficient and sustainable management of groundwater. Assessment the impact of enviromental change such as urbanization which is cause land use change is necessary to determine of groundwater recharge. We can measure groundwater recharge directly or indirectly in specific area and short periods of time. The main focus is estimation of groundwater recharge due to distributed land-use, soil texture, topography, groundwater level, and hydro-meteorological conditions. Spatial variation in recharge due to distributed land-use, soil texture, topography, groundwater level, and hydro meteorological conditions are very important parameters which should be accounted for in recharge estimation. The estimation of groundwater recharge, surface runoff and evatranspiraton can be figured and calculated by WetSpass. WetSpass stands for Water and Energy Transfer between Soil, Plants and Athmosphere for long-term average recharge input for spatial variability.

Main Concept of Simulation ProceduresThe schematic of water balance is modelled in the WetSpass depend on the resolution of raster cell. The process in each part of cell are et in a cascading way.

Fig.1 Simple schematic of water balance assume in WetSpass

The simulation procedures begin with WetSpass before use MODFLOW to stimulate groundwater flow. The resulting spatially groundwater recharge output from WetSpass is the used as an input for MODFLOW. Groundwater depth in WetSpass are used for input for the next iteration until convergence. Convergence is said to be reached when the changes in computed heads occurring from successive iterations is less than the value specified by the user.If the convergence is fulfilled, the groundwater head will be displayed. We also have to determine the boundary condition of seasons, land use and soil type before run WetSpass model. Because WetSpass also can be applied to analyze the effect of topography input, it can perform the analysis of daily runoff as reaction of the catchment of rainfall was performed. Fast and slow discharge coefficients were calculated from WetSpass output data as these discharge coefficients were related to the total surface runoff and groundwater recharge respectively.

Fig.2 Process of groundwater recharge modeling on WetSpass and MODFLOW

WetSpass was completely integrated in the GIS ArcView as a raster model. Several parameters including land use, soil type, run off coefficient are used as input. Here is several input from GIS we need for modelling at WetSpass

Table 1. ArcView input for WetSpass

Table 2. Input files for MODFLOW in (.ascii) format

Model DescriptionIt is a physically based model for the estimation of long-term average spatial patterns of groundwater recharge, surface runoff and evapotranspiration employing physical and empirical relationships. The water balance components of vegetated, bare soil and imprevious surfaces in WetSpass are calculated as follows :

ET raster=av ET v+as E s+ao Eo+ai Ei

Sraster=av Sv+as Ss+ao So+ai Si

Rraster=av Rv+as R s+ao Ro+ai Ri

Which are ETraster, Sraster, Rraster are the total evapotranspiration, surface runoff, and groundwater recharge of a raster cell respectively, each having a vegetated, bare-soil, open-water and impervious area component denoted by av, as, ao, and ai. Precipitation is taken as the starting point for the computation of the water balance of each of the above mentioned components of a raster cell, the rest of the processes (interception, runoff, evapotranspiration, and recharge) follow in an orderly manner.

The water balance for a vegetated area depends on the average seasonal precipitation (P), interception fraction (I), surface runoff (Sv), actual transpiration (Tv), and groundwater recharge (Rv) all with the unit of [LT-1], with the relation given below:

P=I +Sv+T v+Rv

Surface runoff (Sv) is calculated in relation to precipitation amount, precipitation intensity, interception and soil infiltration capacity.

Sv=CHor Sv− pot

Initially the potential surface runoff (Sv - pot) is calculated as :

Sv−pot=C sv (P−I )

Which in Csv is a surface runoff coefficient for vegetated infiltration areas, and is a function of vegetation, soil type and slope. Where CHor is a coefficient for parameterizing that forms part of a seasonal

precipitation contributing to the overland flow. CHor for groundwater discharge areas is equal to 1.0 since all intensities of precipitation contribute to surface runoff.

The calculation of seasonal evatranspiration is obtained from open water evaporation and vegetation coefficient

T rv=c Eo

Trv = the reference transpiration of a vegetated surface [LT-1], Eo = potential evaporation of open water [LT-1] and c= vegetation coefficient [–].

This vegetation coefficient can be calculated as the ratio of reference vegetation transpiration as given by the Penman-Monteith equation to the potential open-water evaporation, as given by the Penman equation:

C=1+ γ

∆

1+ γ∆

(1+r c

ra)

Which γ is psychrometric constant [ML-1T-2C-1], Δ is slope of the first derivative of the saturated vapor pressure curve (slope of saturation vapor pressure at the prevailing air temperature) [ML1T-2C-1], rc =

canopy resistance [TL-1] and ra = aerodynamic resistance [TL-1] given by

ra=1

k2ua( ln( za−d

z0 ))2

Which k is the Von Karman constant (0.4) [–], ua is the wind speed [LT-1] at measurement level za = 2m, d is the zero-plane displacement length [L] and zo is the roughness length for the vegetation or soil [L].For vegetated groundwater discharge areas, the actual transpiration (Tv) is equal to the reference transpiration as there is no soil or water availability limitation

T v=T rv , if (G d−ht)≤ Rd

Which Gd, is groundwater depth [L], ht is the tension saturated height [L] and Rd is the rooting depth [L]. The last component, the groundwater recharge, is then calculated as a residual term of the water balance can be calculated as follow :

R v=P−Sv−E T v−Es−I

ETvv is the actual evapotranspiration [LT-1] given as the sum of transpiration Tv and Es (the evaporation from bare soil found in between the vegetation). The spatially distributed recharge is therefore estimated from the vegetation type, soil type, slope, groundwater depth, and climatic variables of precipitation, potential evapotranspiration, temperature, and wind-speed. WetSpass recharge outputs can be used as an input for the groundwater model like MODFLOW. MODFLOW is an extremely versatile finite-difference groundwater model that simulates three-dimensional groundwater flow through a porous medium [6].

Ss=∂ h∂ t

= ∂∂ x ( K xx

∂h∂x )+ ∂

∂ y ( Kxx∂h∂ y )+ ∂

∂ z (Kxx∂ h∂ z )−W

Ss is the specific storage of the porous material [L-1], Kxx, Kyy and Kzz are hydraulic conductivity along the x, y and z coordinate axes, which are assumed to be parallel to the major axes of hydraulic conductivity [LT-1], h is the potentiometric head [L] and W is volumetric flux per unit volume, representing sources and/or sinks of water [L3T-1] and t is time [T]. The ground water flow equation is solved using the finite-difference approximation.

References :[1]Rwanga, Sophia S. 2013. A Review on Groundwater Recharge Estimation Using Wetspass Model. International Conference on Civil and Enviromental Engineering. 156-160

[2]Bateelan, Okke & De Semdt, Florimond. 2001. WetSpass : a flexible distributed recharge methodology for regional groundwater modelling. Proceedings of a symposium held during the SixthIAHS Scientific Assembly. The Netherlands. 29. 11-17