A GIS Based Water Balance Study of Africa - Physical Land Resources

19GROUNDWATER RECHARGERecharge refers to the process where water moves downward from surface water to groundwater(vadose) zone below plant roots. WetSpass calculates groundwater recharge as a residual termof the water balance. That is:R v = P-S v - ET v -I (2.13)ET v is the actual evapotranspiraton [LT -1 ] given as the sum of transpiration T v and E s (theevaporation from the bare soil in between the vegetation)By applying the above equations and relations spatially distributed recharge can thus beestimated from the vegetation type, soil type, slope, groundwater depth, precipitation, potentialevapotranspiration, temperature and wind speed.WATER BALANCE FOR BARE SOIL, OPEN WATER AND IMPERVIOUS SURFACESRecharge is estimated from the water balance for bare soil as:P = S s + E s + R s (2.14)Where P is the precipitation , S s, is the surface runoff, E s is the actual evapotranspiration, and R s isthe groundwater recharge. All have the unit [LT- 1 ] Equation (2.14) is the same as Equation (2.4)except that there is no interception and transpiration term. The ET v in this case becomes E s . Thesurface runoff is simulated in a similar way to the vegetated area fraction in two stages. In the