Irrigation Effects on Soil Moisture Analysis in ERA-Interim Reanalysis

In the ERA interim atmospheric re-analysis system, model predictions of the soil moisture content are corrected with actual measurements by re-analysis. Over the period 1990-2014, these corrections show a clear global pattern of mean soil moisture additions in many areas, which cover a significant part of the total soil moisture variability. The spatial and temporal patterns of these soil moisture additions are compared to irrigation water demand and precipitation bias, which could both be causing the soil moisture additions.

In irrigated areas, the annual means and cycles of these soil moisture additions correlate well with irrigation, while these correlate less with the precipitation bias. Therefore, it is concluded that in irrigated areas, it is more likely that the soil moisture additions are caused by irrigation than by the precipitation bias. In non-irrigated areas, a weak statistical relation between soil moisture additions and precipitation bias is present. As irrigation influences the water balance in atmospheric reanalysis systems, it is recommended to include this process in the reanalysis models. Moreover, as irrigation has an influence on the local and regional atmosphere, this influence should be taken into account when interpreting atmospheric data over strongly irrigated areas.