The key indicators that can be monitored refer to the hydrological cycle. Therefore, a water quantity monitoring network generally consists of measurement of the rainfall, the streams flow and the evapotranspiration. Streams flow is composed of the runoff, which is the water that goes directly to the streams after a rainfall and base flow which are the outputs of the groundwater system.
Groundwater storage variation is the balance between infiltration and aquifer resurgences which is actually the base flow in the streams (may in other watersheds). It is much more complex to estimate, although it may be theoretically estimated from the general balance; Actually, the groundwater system does not correspond to the watershed limit and geology make that the groundwater reservoir is not necessarily uniform and continuous. Only complex modelling tools can generally partially help understanding the interrelation between infiltration and resurgences.