The document discusses methods for estimating groundwater recharge using the WetSpass and MODFLOW models. WetSpass is a physically based model that uses inputs like land use, soil type, and climate data to calculate spatial patterns of evapotranspiration, surface runoff, and groundwater recharge. The recharge output from WetSpass is then used as input for the MODFLOW groundwater flow model. Together, WetSpass and MODFLOW allow for the simulation of spatially variable recharge and groundwater flow.

1. Estimation of Groundwater Recharge
Using WetSpass and MODFLOW
Putika Ashfar K
February 2016
Introduction
Evaluation 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 Procedures
The 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

2. 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

3. Table 2. Input files for MODFLOW in (.ascii) format
Model Description
It 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 :
𝐸𝑇𝑟𝑎𝑠𝑡𝑒𝑟 = 𝑎𝑣𝐸𝑇𝑣 + 𝑎𝑠𝐸 𝑠 + 𝑎𝑜𝐸 𝑜 + 𝑎𝑖𝐸𝑖
𝑆 𝑟𝑎𝑠𝑡𝑒𝑟 = 𝑎𝑣𝑆 𝑣 + 𝑎𝑠𝑆 𝑠 + 𝑎𝑜𝑆 𝑜 + 𝑎𝑖𝑆𝑖
𝑅 𝑟𝑎𝑠𝑡𝑒𝑟 = 𝑎𝑣𝑅 𝑣 + 𝑎𝑠𝑅 𝑠 + 𝑎𝑜𝑅 𝑜 + 𝑎𝑖𝑅 𝑖
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 + 𝑆 𝑣 + 𝑇𝑣 + 𝑅 𝑣
Surface runoff (Sv) is calculated in relation to precipitation amount, precipitation intensity, interception
and soil infiltration capacity.
𝑆 𝑣 = 𝐶 𝐻𝑜𝑟 𝑆𝑣−𝑝𝑜𝑡
Initially the potential surface runoff (Sv - pot) is calculated as :
𝑆 𝑣 − 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

4. 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
𝑇𝑟𝑣 = 𝑐𝐸 𝑜
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 +
𝑟𝑐
𝑟𝑎
)
Which γ is psychrometric constant [ML
-1
T
-2
C
-1
], Δ is slope of the first derivative of the saturated vapor
pressure curve (slope of saturation vapor pressure at the prevailing air temperature) [ML
1
T
-2
C
-1
], rc
=
canopy resistance [TL
-1
] and ra
= aerodynamic resistance [TL
-1
] given by
𝑟𝑎 =
1
𝑘2 𝑢 𝑎
( 𝑙𝑛(
𝑧 𝑎 − 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
𝑇𝑣 = 𝑇𝑟𝑣 , 𝑖𝑓 (𝐺 𝑑 − ℎ 𝑡) ≤ 𝑅 𝑑
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 − 𝑆 𝑣 − 𝐸𝑇𝑣 − 𝐸 𝑠 − 𝐼
ETv
v
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].
𝑆 𝑠 =
𝜕ℎ
𝜕𝑡
=
𝜕
𝜕𝑥
( 𝐾𝑥𝑥
𝜕ℎ
𝜕𝑥
) +
𝜕
𝜕𝑦
( 𝐾𝑥𝑥
𝜕ℎ
𝜕𝑦
) +
𝜕
𝜕𝑧
( 𝐾𝑥𝑥
𝜕ℎ
𝜕𝑧
) − 𝑊

5. 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 [L
3
T
-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 ofa symposium held during the SixthIAHS
Scientific Assembly. The Netherlands. 29. 11-17