A single layer soil water balance model for estimating deep drainage (potential recharge): An application to cropped land in semi-arid North-east Nigeria

Abstract

The understanding and quantification of groundwater recharge in semi-arid areas are fundamental to sound management of water resources in such areas. A soil water balance model, if designed to adequately represent the physical processes involved, and if carried out with a short enough (daily) time step, can provide realistic estimates of deep drainage (potential recharge) over long periods.We describe a single store (single layer) mass water balance model applicable to semi-arid areas, which recognises the wetting of the near surface during rainfall, with subsequent availability of water for evaporation and transpiration in the days following rainfall. The model allows for the major hydrological processes taking place at or near the soil-vegetation surface including runoff.Model results are presented for North-east Nigeria, for a continuous period of 36 years during which mean annual rainfall was 431 mm (range 321-650 mm) and mean annual modelled deep drainage was 14 mm (range 0-95 mm, with 23 years having zero potential recharge). The modelling results indicate that annual rainfall totals are not the main predictor of annual recharge. The temporal distribution of daily rainfall and the magnitude of the antecedent (pre-season) soil moisture deficit are the strongest determinants of deep drainage at a particular location, in a particular year. Sensitivity analysis of soil and vegetation parameters suggests that deep drainage is most sensitive to water holding capacity and rooting depth. These are key parameters which determine spatial variability of potential recharge.The model is shown to be plausible by examination of the concepts which underlie it, by comparison with field soil moisture measurements, and by the model's ability to represent qualitative observations of crop yield variations from year to year.Future development of the model could include applications to other climatic conditions and the inclusion of other hydrologic processes.

Simulation of Soil Hydrological Components in Chuncheon over 30 years Using E-DiGOR Model

“…Annual drainage rates below 120 cm soil depth ranged from 477.8 to 1565.9 mm and depended on the intensity and frequency of rainfall events and especially soil water storage from the preceding dry periods. A similar trend in drainage was also reported by Eilers et al (2007), Aydin (2008). The long-term mean annual drainage-loss was approximately two times higher than actual soil evaporation.…”

Climate Change and Sustainable Development of Water: Sub-Saharan Africa

“…Contributing to the non-linearity of the relationship between rainfall and recharge, some studies suggest that a certain threshold of rainfall (annual or event-driven) must be reached before any recharge to groundwater occurs. The large inter-annual variability of rainfall, thus, may lead to large inter-annual variability of recharge (Eilers et al, 2007;Edmunds, 2009;Olago et al, 2009). Insignificant recharge is thought to occur when rainfall is generally less than 200 mm yr -1 , though some recharge during individual intense events less than that amount (Scanlon, et al, 2006;Edmunds 2009).…”

“…One study observed recharge taking place approximately every 10 years for a duration of a 3-year pluvial event (Olago et al, 2009). Individual intense events, in general, may be contributing more to recharge than the sum of all daily rainfall events (Taylor and Howard, 1996;Eilers et al, 2007;Owor et al, 2009). The decline of groundwater levels has been observed during periods of sustained annual reduction of rainfall such as during drought.…”

What impact will climate change have on rural groundwater supplies in Africa?

“…Butterworth et al, 1999). While recharge proportions in excess of 10% are possible in some areas with mean annual rainfall below 500 mm, the proportion is generally less than this, typically falling to negligible for areas with rainfall below 200 mm/year (Lerner et al, 1990;Scanlon et al, 2006;Eilers et al, 2007;Edmunds, 2008;WHYMAP, 2008). Figure 4 shows a simplified groundwater resources map for Africa.…”

“…In addition, the permeability and porosity of underlying aquifers can limit the capacity for recharge to be stored. Nonetheless, for the purpose of making a general assessment for the whole of Africa, it is reasonable to assume that negligible recharge occurs in areas with annual rainfall of less than 200 mm (Eilers et al, 2007), and that recharge of up to approximately 50 mm can occur in areas with annual rainfall in the range of 200-500 mm (De Vries & Simmers, 2002;Edmunds, 2008), and greater in areas where rainfall exceeds 500 mm (e.g. Rueedi et al, 2005).…”