Humble opinion on assessment indices for groundwater renewability: Applicability of renewal period and recharge rate

The renewability of groundwater has been a hot topic in the field of groundwater science and engineering in recent years. However, the definition of groundwater renewability has not been uniform, and the assessment indices are also relatively diverse, of which the groundwater renewal period, recharge rate, age and residence time are commonly used. The assessment results of groundwater renewability on the basis of those indices are usually inconsistence with each other. This is because different physical characteristics are considered in the establishment of those indices, though close relationships do exist between them. Based on a case study from Beijing, a comparison is done on the applicability of two closely related indices (renewal period and recharge rate) for assessing the groundwater renewability. The results show that: ① the different conclusions could be reached for the relative strength of groundwater renewability in Beijing's districts and counties depending on the assessment indices used; ② the conclusion with the groundwater recharge rate seems to be more stable compared to the other one; ③ the groundwater recharge rate has more important practical significance being an assessment index for groundwater renewability.