The concept of dual-boundary forcing in land surface-subsurface interactions of the terrestrial hydrologic and energy cycles

Terrestrial hydrological processes interact in a complex, nonlinear fashion. It is important to quantify these interactions to understand the overall mechanisms of the coupled water and energy cycles. In this study, the concept of a dual-boundary forcing is proposed that connects the variability of atmospheric (upper boundary) and subsurface (lower boundary) processes to the land surface mass and energy balance components. According to this concept, the space-time patterns of land surface mass and energy fluxes can be explained by the variability of the dominating boundary condition for the exchange processes, which is determined by moisture and energy availability. A coupled subsurface-land surface model is applied on the Rur catchment, Germany, to substantiate the proposed concept. Spectral and geostatistical analysis on the observations and model results show the coherence of different processes at various spacetime scales in the hydrological cycle. The spectral analysis shows that atmospheric radiative forcing generally drives the variability of the land surface energy fluxes at the daily time scale, while influence of subsurface hydrodynamics is significant at monthly to multimonth time scales under moisture-limited conditions. The geostatistical analysis demonstrates that atmospheric forcing and groundwater control the spatial variability of land surface processes under energy and moisture-limited conditions, respectively. These results suggest that under moisture-limited conditions, groundwater influences the variability of the land surface mass and energy fluxes. Under energy-limited conditions, on the contrary, variability of land surface processes can be explained by atmospheric forcing alone.