Role of topographic gradient on water use performance in a paddy area

Flooding irrigation management in paddy areas requires copious volumes of water, which are highly variable mainly according to soil features, groundwater table depth and climate conditions. Irrigation efficiencies ranging between 20% and 60% are commonly reported for field-scale studies. Despite researches investigating the water use efficiency (WUE) of paddy areas at larger spatial scales are scarce, there is evidence that water performance indicators may increase with increasing spatial scale because of water reuse. In paddy areas, topography is one of the factors activating water exchanges and reuse between rice fields. In particular, lower paddies may receive seepage and runoff from paddies located upslope. Very few studies in the literature addressed this topic so far. This study investigated the water balance terms and the water use performance of a group of four rice fields located in Lombardy and characterized by different elevations (A ≈ B > C > D) during two years (2015-2016). The main outcomes of the study are: i) paddy fields on a slope have interdependent WUE, with lower values at the top than at the bottom of the toposequence (WUE even greater than one may be observed for downslope fields); (b) WUE of none of the fields in a toposequence can be considered representative of the whole area: thus, to quantify the WUE of a series of paddies on a slope, the monitoring scale must be enlarged to include all the fields in the toposequence; (c) when groundwater is shallow, WUE is strongly dependent on the groundwater level both at the field-scale and at the slope-scale: significant variations in water fluxes and WUE may be observed from one year to the next as a consequence of fluctuations in the groundwater table depth.