Insights From a Multi-Method Recharge Estimation Comparison Study

Abstract

Although most recharge estimation studies apply multiple methods to identify the possible range in recharge values, many do not distinguish clearly enough between inherent uncertainty of the methods and other factors affecting the results. We investigated the additional value that can be gained from multi-method recharge studies through insights into hydrogeological understanding, in addition to characterizing uncertainty. Nine separate groundwater recharge estimation methods, with a total of 17 variations, were applied at a shallow aquifer in northwest Ethiopia in the context of the potential for shallow groundwater resource development. These gave a wide range of recharge values from 45 to 814 mm/a. Critical assessment indicated that the results depended on what the recharge represents (actual, potential, minimum recharge or change in aquifer storage), and spatial and temporal scales, as well as uncertainties from application of each method. Important insights into the hydrogeological system were gained from this detailed analysis, which also confirmed that the range of values for actual recharge was reduced to around 280-430 mm/a. This study demonstrates that even when assumptions behind methods are violated, as they often are to some degree especially when data are limited, valuable insights into the hydrogeological system can be gained from application of multiple methods.

Figure 1

Location map of the study area.

Figure 2

Monthly median, 10th and 90th percentile rainfall, and mean maximum and minimum temperatures as measured (1987‐2017) by the NMA at the Dangila weather station.

Figure 3

Conceptual model of the study site.

Figure 4

Plot showing the relationship between annual rainfall and annual recharge in Ethiopia based on 102 recharge estimates from 49 studies across the country. S = standard error, R ² (adj.) = adjusted coefficient of determination. The Tigray, Afar, Dire Dawa group has semi‐arid climate and highly heterogeneous geology ranging from Precambrian crystalline to Mesozoic sandstones and limestones to Quaternary volcanics, generally overlain by leptosols with sparse and herbaceous vegetation. Rift Valley and central Ethiopia have subtropical highland and tropical savanna climate with Quaternary volcanic geology, highly heterogeneous soils and rainfed cropland and mosaic forest and grassland. The Lake Tana Basin has a tropical highland monsoon climate and Cenozoic volcanic rocks overlain by luvisols or vertisols closer to the lake with mosaic cropland/grassland/shrubland/forest (Tefera et al. 1996; Peel et al. 2007; Arino et al. 2012; Jones et al. 2013).

Figure 5

Median annual recharge estimates from all the techniques. The error bars give the interannular recharge range. T‐M = Thornthwaite‐Mather method of runoff or AET estimation. R‐S = Remote sensing method of AET estimation.

Figure 6

Comparison of the sensitivity of each recharge estimation method to ±10% adjustment in measured input data and modeling parameters (left) and range of uncertainty when the maximum likely deviations are applied (right).