A Critical Analysis of the Cumulative Rainfall Departure Concept

Document Type


Publication Date


Digital Object Identifier (DOI)



Evaluation of trends in time-series, such as precipitation or ground water levels, is an essential element in many hydrologic evaluations, including water resource studies and planning efforts. The cumulative rainfall departure (CRD) from normal rainfall is a concept sometimes utilized to evaluate the temporal correlation of rainfall with sur- face water or ground water levels. Permutations of the concept have been used to estimate recharge or aquifer stora- tivity, and in attempts to explain declining ground water levels. The cumulative departure concept has hydrologic meaning in the short term, as a generalized evaluation of either meager or abundant rainfall, and when utilized in con- nection with a detailed water budget analysis can be used in a predictive fashion. However, the concept can be mis- applied if extended over lengthy periods. Misapplication occurs because of several factors including the separation of the mean and median in nonnormal distributions, how the choice of beginning and end points of the data can affect the results, the lack of consideration that above-average rainfall can reset the hydrologic system without mathemati- cally eliminating the accumulated deficit, and the lack of support for the necessary inference that rainfall events and hydrologic levels widely separated in time are linked. Standard statistical techniques are available to reliably deter- mine trends and can provide rigorous statistical measures of the significance of conclusions. Misuse of the CRD con- cept can lead to erroneous and unsupported conclusions regarding hydrologic relationships and can potentially result in misguided water resource decision-making.

Was this content written or created while at USF?


Citation / Publisher Attribution

Groundwater, v. 42, issue 6, p. 935-938