Ecosystem Viltality Water Quantity (WQT) Groundwater Storage Depletion(GwSD)

Groundwater storage depletion measures changes in the availability of water stored in underground aquifers (Konikow and Kendy 2005). This can be directly estimated using records of groundwater level obtained from observation wells or via the indirect proxy of aquifer compression resulting from groundwater over-exploitation, or using data derived from Gravity Recovery and Climate Experiment (GRACE) satellites.

Data Preparation:

Identify areas with a potential GW depletion problem:

Areas with potential depletion problems can be identified by the following methods (following Vrba and Lipponen (2007)):

• Areas with a high density of production wells: Groundwater level declines are strongly associated with an increase of pumping costs or loss of spring or production well yields, which can indicate groundwater depletion in areas where many wells are exploiting an aquifer. Two alternatives for identifying water level declines are: 1) to detect a consistent and gradual downward trend of water level from a well monitoring network (when available) or 2) to compare the groundwater level at wells drilled at different times (i.e., compare water level evolution using nearby wells, but drilled at different times: 1960s, 1970s, etc.). For the latter, it is fundamental to have a well inventory that can provide information about the well construction and hydraulic parameters of the aquifer. (In basins large enough, GRACE data may help establish depletion.)

• Change of base flow: In many areas, rivers and other surface water bodies receive an important proportion of their water from groundwater base flow. Drastic reduction of this groundwater flux and loss of base flow can be associated with groundwater depletion. In this case, the monitoring of river flow is important. An indirect indication of reduction of base flow can be established when phreatic vegetation or wetlands suffer notable changes.

• Change of groundwater quality characteristics: Although the physical-chemical properties of water can vary throughout the aquifer, in conditions of regular exploitation, drastic changes in groundwater quality are not expected (including stable isotope composition). Therefore, changes in age and origin of groundwater at specific locations in the aquifer can be an indication of groundwater depletion.

• Land subsidence: At some localities, groundwater exploitation from thick sedimentary aquifer-aquitard systems has been accompanied by significant land subsidence. In this case, land subsidence can be used as an indirect indicator of unsustainable groundwater exploitation.

As Vrba and Lipponen (2007) notes, care must be taken when evaluating GW depletion because it is also subject to natural and seasonal fluctuations from the influence of climatic conditions and aquifer characteristics. Sometimes, groundwater storage depletion may also be associated with a long transient evolution from one steady state to another and does not necessarily represent a problem of unsustainable exploitation of an aquifer. The most difficult problem in aquifers that are subjected to exploitation is distinguishing permanent and regional depletions from only temporal and local interferences caused by the proximity of production wells.

Calculation in FHI Toolbox:

$$ GwSD = \left( 1 - \frac{\sum_{}^{}a}{A} \right)*100 $$

Where a is the area with depletion problems identified, and A is the sub-basin/basin area being studied.

Once the area with depletion problems has been identified, variable a in the formula above may be obtained from local studies aimed at estimating aquifer depletion.