Deciphering the declining runoff in the Thaya river basin

This study aims at understanding the changes in the water balance in the Thaya river basin over the past 40 years. The Thaya River is one of the tributaries to the Danube basin with a catchment area of more than 13 000 km². A number of hydroclimatic variables related to runoff were examined by a trend analysis based on Theil-Sen regression and Mann-Kendall tests for the two periods 1981–2020 and 2001–2020. The latter period was selected because it allows analysis of several relevant variables derived from the Moderate Resolution Imaging Spectroradiometer (MODIS). These variables ecompass snow cover, leaf area index and land surface temperature based actual evapotranspiration.

With our analyses we confirm previously found increasing trends in air temperature, ETo, and no trends in precipitation. We also found a consistent increase of ET during spring months and indication of summer decrease (not statistically significant). This change was associated with a significant increase of spring vegetation development followed by summer stagnation. We identified a significant trend decline in runoff, mainly in the upland sourcing areas. The correlation analysis reveals a different behavior along the elevation gradient, with evapotranspiration in the uplands being limited by energy and in the lowlands by water, especially in spring. During summer, however, the entire basin is often water-limited, with a more pronounced limitation in the lowlands. According to attribution analysis for the past 20 years, the significantly decreasing runoff is driven primarily by non-significantly decreasing precipitation, significantly increasing air temperature and vapor pressure deficit. Global radiation and wind speed affect the runoff only to a very limited extent. We conclude that complex adaptation measures reflecting the site specificity and elevation gradient are needed to sustain the water dependent sectors operating in the region facing increasing aridity.