Evapotranspiration over agroforestry sites in Germany

Abstract

In the past few years, the interest in growing crops and trees for bioenergy production has increased. One agricultural practice is the mixed cultivation of fast-growing trees and annual crops or perennial grasslands on the same piece of land, which is referred to as one type of agroforestry (AF). The inclusion of tree strips into the agricultural landscape has been shown on the one hand to lead to reduced wind speeds and higher carbon sequestration above ground and in the soil. On the other hand, concerns have been raised about increased water losses to the atmosphere via evapotranspiration (ET). Therefore, we hypothesise that short rotation coppice agroforestry systems have higher water losses to the atmosphere via ET compared to monoculture (MC) agriculture without trees. In order to test the hypothesis, the main objective was to measure the actual evapotranspiration of five AF systems in Germany and compare those to five monoculture systems in the close vicinity of the AF systems. We measured actual ET at five AF sites in direct comparison to five monoculture sites in northern Germany in 2016 and 2017. We used an eddy covariance energy balance (ECEB) set-up and a low-cost eddy covariance (EC-LC) setup to measure actual ET over each AF and each MC system. We conducted direct eddy covariance (EC) measurement campaigns with approximately 4 weeks' duration for method validation. Results from the short-term measurement campaigns showed a high agreement between ETEC-LC and ETEC, indicated by slopes of a linear regression analysis between 0.86 and 1.3 (R2 between 0.7 and 0.94) across sites. Root mean square errors of LEEC-LC vs. LEEC (where LE is the latent heat flux) were half as small as LEECEB vs. LEEC, indicating a superior agreement of the EC-LC set-up with the EC set-up compared to the ECEB set-up.