Modelling Groundwater Recharge of Otukpo using Modified Soil Moisture Balance Methodology.

Abstract

In this paper, groundwater recharge in Lower Benue River Basin is estimated using a modified daily soil moisture balance based on a single soil water store for a climate classified as tropical with distinct dry and wet seasons in the middle belt part of Nigeria. Soil properties like field capacity, permanent wilting point, readily available water, actual and potential evapotranspiration, soil moisture deficit were all estimated and deployed in the model which algorithm was developed using Python programming language, hence, the name modified soil moisture balance model. Runoff is estimated using runoff matrix and runoff coefficients, which depends on rainfall intensity and soil moisture deficits. A new components, near surface storage is used to represent continuing evapotranspiration on days following heavy rainfall even though the soil moisture deficit is high. Groundwater recharge is estimated for the basin where cassava and yam are the commonly cultivated vegetable crops. Meteorological data such as rainfall, daily minimum and maximum temperature, radiation, relative humidity all for the period of 2008 to 2018 were used in the model analysis. The model recorded annual groundwater recharge which varied from 38.119mm in 2017 water year (just 3.6% of annual rainfall for the year) to 333.35mm in 2009 water year which is 20.01% of annual rainfall for the year). The highest annual rainfall depth was also observed in the year 2009 as 1665.4mm, with the lowest annual rainfall depth, 1062.4mm also observed in the year 2017. The annual rainfall ranged from 322.04mm in the year 2015, a 32.16% of annual rainfall for the year to 935.56mm in the year 2008, a 58.17% of annual rainfall for the year. The lowest actual evapotranspiration AE was also observed in 2017 as against the highest in 2012. The AE ranged from 583.84mm in 2017 to 721.39mm in 2012. The model gave a simplified method of groundwater recharge estimation as well as runoff depth coupled with rainfall-runoff relationship.