Lischeid, G; Frei, S; Huwe, B; Bogner, C; Lüers, J; Babel, W; Foken, T: Catchment Evapotranspiration and Runoff in Foken, T.: Energy and Matter Fluxes of a Spruce Forest Ecosystem, Springer(Ecological Studies Vol. 229), 355-375 (2017), doi:10.1007/978-3-319-49389-3_15 [Link]

The interplay between precipitation and evapotranspiration determines the input into the hydrological system of a catchment. Annual values of precipitation, evapotranspiration, and runoff measured at the catchment outlet for the 2002–2009 period were available. Annual precipitation clearly surmounted the sum of evapotranspiration and runoff. Part of the observed discrepancy might be due to the heterogeneity of precipitation and evapotranspiration within the catchment which has not been studied in sufficient detail. Annual evapotranspiration fluxes were remarkably constant during this period, whereas precipitation and runoff exhibited much larger interannual variability.

Short-term dynamics of soil matrix potential, groundwater head, and discharge were studied using principal component analysis. About 19 % of the spatial variance of soil matrix potential in the soil in a mature spruce forest was ascribed to the effect of spatially varying root water uptake. In addition, the analysis illustrated the effect of increasing damping and delay of the input signal with increasing depth. That of the runoff at the catchment outlet ranged between that of soil matrix potentials at 40 cm depth, pointing to near-surface runoff generation. This gives clear evidence for the respective flowpath during stormflow. Thus, input signals imposed by heavy rainstorms reach the catchment outlet within a few hours. In contrast, changes of evapotranspiration become visible at the catchment outlet only with a few years’ time delay via corresponding changes of groundwater recharge that are transferred through the aquifer. In hydrological and groundwater models, usually little attention is paid to the mutual interplay between evapotranspiration and root water uptake on the one hand and hydrological processes, especially in shallow groundwater areas. Sound assessments of effects of land use and climate change, however, need to account for that in more detail.

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