Groundwater exploitation: safe yield versus sustainable yield: example of the Aynalem well field, Mekelle

The concept of safe yield is often being used to refer to the amount of groundwater that can be exploited from a well or from an aquifer. Although many definitions have been given for this concept, since its introduction in 1915 by Lee, it could be defined as the quantity of groundwater that can be withdrawn on a permanent basis, whereby a new dynamic equilibrium is reached, and the aquifer storage has been brought to a new constant level, thus preventing the further decrease of hydraulic heads. This condition can only be reached by an increased recharge and a decreased discharge. Theis (1940) gave a name to the sum of both: the capture. In overexploited groundwater systems (beyond safe yield), also part of storage is captured, resulting in continuously lowering water levels. The safe yield is frequently estimated from the natural aquifer recharge, based on the reasoning that total recharge, or a fraction thereof, can be withdrawn without changing the system. This reasoning is completely false, and has been termed by Bredehoeft (1997; 2002): “The water budget myth”. Usually, for unconfined aquifers, recharge is assumed to be constant, as being delivered by precipitation (Bredehoeft, 1997) and the capture is equal to the decreased discharge. When starting up a new groundwater exploitation, water is mainly derived from storage. As evolution progresses towards a new equilibrium, the contribution of storage decreases, while capture increasingly delivers the pumped water. Ultimately, a new dynamic equilibrium is reached, with constant, but lowered groundwater levels. This decrease in discharge implies that springs, surface waters, … may have dried up. The time for this new equilibrium to establish (provided it is reached), may be long. Moreover, the localisation of the new well field matters for determining this dynamic response: the safe yield of an aquifer cannot be quantified without identifying concrete pumping stations. This important condition is usually neglected. This safe yield is not necessarily sustainable. The definition of the sustainable yield of an aquifer is not an exclusively hydrological problem. It needs to be based on the acceptability of a certain lowering of groundwater levels. In other words, different options are possible. Next to hydrological thoughts, also other environmental concerns (e.g. fauna and flora) have to be considered, and moreover social and economic factors are important. Sustainable groundwater development is a multi-dimensional problem, which cannot be solved by hydrogeologists alone. Groundwater models are used to study the dynamic response of groundwater systems to specific stresses, including groundwater exploitation. However, hydrogeologists need decisions about options, to be made by other stakeholders, as an input for their models. For example: is this certain spring important to society, for nature conservation, … or can it be sacrificed for the sake of an increased water supply from a well field?. Without pretending to solve the question of sustainability of the Aynalem well field, this example was studied by calculating natural groundwater recharge and by (an attempt to) quantify the drawdown around the well field using a mathematical model