Water Deficit Duration and Severity Analysis Based on Runoff Derived from Noah Land Surface Model

Abstract

The identification and prediction of drought events depend on the integrity of the dataset employed. Streamflow is a good indicator of surface water availability and has dominated the literature on frequency analysis of hydrological droughts and water management. However, gauged measurements are impaired by climate and land use changes, especially in large, modified watersheds. Hence, their use in drought prediction is limited because they may violate the assumption of stationarity unless a naturalized observation series is obtained. In this paper, a land surface model is used to generate runoff in the Rio Grande/Río Bravo del Norte basin. Land use land cover is kept constant so that changes are a result of climatological variations. The river threads across several climatic zones; therefore the basin is divided into different regions for analysis. Using statistical theory of runs, water deficit duration and severity and drought interarrival time are extracted from 3-month Standardized Runoff Index series for each region and for the whole basin. Copulas are used to develop joint distribution functions of water deficit duration and severity. Nine copulas, two from the extreme value family and seven from the one-parameter families of the Archimedean copulas, are tested. Different copulas are deemed suitable for each region because they are subject to different climatologic conditions, which affect the nature of deficit events, especially at the extremes. Conditional probability models of duration and severity are developed and compared for two regions—one experiencing alpine climate at the headwaters and one in the semiarid portion of the basin. Further, conditional probabilities derived from model runoff are compared with that based on observed precipitation. Finally, univariate and conditional return periods based on duration and severity are computed for the basin and their significance in long-term water resources planning is discussed.