Water Balance for Mbagathi Sub-Catchment

J.M. Nyika, G.N. Karuku, R.N. Onwonga



Increasing water demands with limited supplies is a concern for agencies charged with its provision while observing equity and fairness. This paper aimed at calculating a simple water balance byequating water supplies to the demands in Mbagathi sub-catchment. Groundwater recharge was estimated using the soil water balance method while surface water supplies used Mbagathi river discharge data from stream-flow gauge stations 3AA04, 3AA06 and 3BA29. Survey data collected in 2015 using snowballing approach was used to quantify water demands. Change in groundwater storage in 2015 and 2014 was significantly (p≤0.05) lower at -0.4 and 1.3 million m3 respectively, compared to 2013, 2012, 2011 and 2010 at 9.9, 11.9, 14.6 and 22.6 million m3, respectively. Reductions in groundwater storage from 2010-2015 were attributed to rise in demand, in efficient use, climatic variations characterized by low rainfall to recharge aquifers and limited exploitation of polished wastewater in the study area. Unsustainable water availability amidst over-reliance on groundwater and dominance in domestic and agricultural uses was observed in Mbagathi sub-catchment necessitating drastic water management measures. The study concluded that adopting water use efficient practices such as water harnessing, water re-use, soil and water conservation measures could ease pressure on existent supplies.

Keywords: Groundwater; surface water; water demand; water supplyMbagathi sub-catchment