Evaluating vegetation regeneration rates after drought

Abstract

Drought events have accelerated and intensified in recent decades. In the 21st century, the number and duration of severe drought events are expected to increase even further. The impact on crops and natural vegetation has already become more pronounced over the last years, as crop yield losses increase and forest fires become more frequent. This creates increasing pressure on water demand and food security globally. Therefore, it is important to understand not only the impact of droughts on vegetation but also the regeneration of vegetation in post-drought periods. In this study, vegetation regeneration rates in Nebraska (United States) are determined using the Standardized Precipitation Evapotranspiration Index (SPEI) and the Enhanced Vegetation Index (EVI). The results show that the correlation coefficient between the SPEI and EVI is generally high for grassland (0.8 - 1.0) and low for cropland (∼ 0.2). The time shift, days between the two datasets at the highest possible correlation coefficient, is between 75 and 125 days for grassland which is mainly dependent on the local topography which is the main driver of groundwater flow and therefore hydraulic gradients in the area. Various factors, including the water table, rooting depth, groundwater recharge delay, recharge area, local depressions filled with water, as well as soil moisture, contribute to deviations in time shifts and recovery times within the study area. The average recovery time of grassland is approximately 3.97 months (121 days), while for cropland it is 3.64 months (111 days). The drought timing is shown to be important in the recovery time. The later a drought ends in the growing season, the longer the vegetation regeneration time. Between the drought time or intensity and the vegetation regeneration time, no significant correlation was found, as soil moisture presumably dominates the drought recovery time. The results of this study provide insight into the vegetation recovery time of both grassland and cropland. Understanding these dynamics is crucial for enhancing large-scale hydrological models, such as PCR-GLOBWB 2.0, by incorporating accurate estimates of vegetation water demands during post-drought periods. As soil moisture is projected to decline further in the future, this research highlights the significance of acknowledging extended periods for drought recovery, thereby contributing to a more comprehensive assessment of the potential impacts of drought on vegetation regeneration for better sustainable water resource management.