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.

Global Isotope Hydrogeology―Review

“…For example, Taylor, Todd, et al () compare precipitation observations in semiarid central Tanzania to nearby well water levels and show that the (i) wettest rainy seasons have the highest recharge ratios and (ii) low‐intensity rainfalls contribute negligible recharge. Other works in humid tropical and arid regions formed similar conclusions: recharge derives disproportionately from intensive rainfall (Harrington et al, ; Owor et al, ; Taylor & Howard, , ), and recharge ratios tend to be higher in rainier areas (e.g., Walker et al, ; Table ). By 2050 about half the global population will live in the tropics, meaning that developing accurate projections of climate change impacts on groundwater replenishment may help develop plans to meet rising water demands in sustainable ways.…”

Development of a Hydrogeological Conceptual Model for Shallow Aquifers in the Data Scarce Upper Blue Nile Basin

“…A multi-method recharge assessment was conducted using data from the field investigations, from both the citizen science and formal hydrometeorological monitoring, and from further published sources, e.g., large-scale recharge maps [40]. Recharge estimation methods applied included baseflow separation, soil moisture balance, water table fluctuation, chloride mass balance, basin water balance, physically based modelling, and an empirical rainfall-recharge relationship.…”

“…The different recharge assessment methods provided a wide range of recharge estimates from 45-815 mm/a (millimetres per annum) [40]. This wide range can be explained by differences in what the calculated "recharge" actually represents for particular methods and the spatial and temporal scales that the method considers.…”

“…Research has shown that local communities can collect high-quality hydrometeorological monitoring data for assessing resource seasonality and availability [38]. Multi-method recharge assessments are recommended for assessing the renewability of the shallow groundwater resource in addition to providing insights into the conceptual model [40]. Applying hydrochemistry, stable isotope, and radon-222 analyses in combination greatly aids conceptual model development.…”

“…This paper is part of a series derived from research at the study site since 2014. Previous publications include: confirming the data quality from the citizen science hydroclimate monitoring programme [38], conducting pumping tests in shallow hand-dug wells [39], insights gained from a comparison of recharge estimation techniques [40], and a discussion on the potential for shallow aquifers to sustain small-scale irrigated agriculture [21]. While this paper draws on those earlier works, the majority of the investigations and development of the conceptual model are presented here first.…”