Extreme Precipitation Years and Their Occurrence Frequency Regulate Long‐Term Groundwater Recharge and Transit Time

Abstract

Core Ideas Long‐term groundwater recharge in semiarid regions was investigated. New deep drainage and water table schemes were added to Community Land Model. Most groundwater recharge at depth occurs in wet years. Frequency of wet years in a period controls the groundwater recharge in that period. Deep drainage, defined as the water flux below the plant root zone, is potential groundwater recharge in arid and semiarid regions but remains poorly understood. This study aims to explore long‐term (1901–2015) deep drainage (at the 75‐m depth) in a semiarid region with deep unsaturated zone (>90‐m depth). For this purpose, a winter wheat (Triticum aestivum L.) field on the Loess Plateau of China was investigated using the Community Land Model (CLM) version 4.0. A new water table scheme was added to CLM version 4.0 to better simulate the water table depth. We found that wet years (with precipitation >650 mm yr−1) are the main source of recharge to the deep soil (e.g., >75‐m depth). The average response time of recharge to a wet year was 8.3 yr to 75‐m depth, which is only 9% of the response time averaged across all non‐wet years in the period. Our results also indicate that the frequency of wet years rather than the average annual precipitation amount plays a more important role in generating deep drainage and groundwater recharge. The results of this study provide new insight into the long‐term groundwater recharge in regions with a deep unsaturated zone.