Exploring scale-effects on water balance components and water use efficiency of toposequence rice fields in Northern Italy

Abstract

Water use efficiencies (WUEs) between 20% and 60% are commonly reported for single rice paddies. When larger spatial domains are considered, higher WUE than minimum values observed for individual fields are expected due to water reuse. This study investigates scale-effects on water balances and WUEs of four adjacent rice fields located in Northern Italy and characterized by different elevations (A ≅ B + C > D). Water balance terms for the paddies were quantified during the agricultural season 2015 through the integrated use of observational data and modelling procedures. Following a Darcy-based approach, percolation was distinguished from net seepage. Results showed net irrigation of about 2,700 and 2,050 mm for fields A and B, and around 640 and nearly 0 mm for C and D. WUE of A, B, C and D amounted, respectively, to 21, 28, 66 and >100%. Values for C and D were due to less permeable soils, to seepage fluxes providing extra water inputs and to the shallow groundwater level. When the group of paddies ACD was considered (B was not included since it was separated by a deep channel), net irrigation and WUE were found to reach 1,550 mm and 39%, confirming the important role of water reuses in paddy agro-ecosystems.

A Comprehensive Modelling Approach to Assess Water Use Efficiencies of Different Irrigation Management Options in Rice Irrigation Districts of Northern Italy

“…Moreover, half of the WFL rice surface is located in areas characterised by a shallow (less than 50 cm from the surface) groundwater table. Furthermore, 1800 mm is well within the range of irrigation requirement values measured in the few studies conducted in Lomellina (e.g., [10]).…”

“…In order to better characterize paddy soils, a detailed soil survey was conducted in a nearby rice farm characterized by soils similar to those found in the pilot district [10]. Undisturbed soil samples taken at different depths from five soil profiles were analyzed, and for each soil sample texture and retention properties were determined.…”

“…When soils become heavier (from loam to clay-loam), fields are placed in central position within toposequences, and groundwater table is shallower (within 50 cm from the soil surface), WUE may rise to 60%. Finally, for fields at the bottom of paddy toposequences, WUE even higher than 100% may be observed due to lateral fluxes coming from upper fields and very shallow groundwater levels [10,11]. In this last study, the authors demonstrate that when paddies are placed on a slope, field position within the toposequence is more important in determining WUE than the irrigation method adopted.…”

“…Results showed similar WUEs for the two techniques, close to 20% (mean values over two years). In recent studies carried out in Pavia for single flooded paddies, WUEs of 20-25% were measured on average-textured soils (from sandy-loam to loam) and groundwater table from 50 to 100 cm from the soil surface during summertime; very low or irrelevant WUE differences were found between WFL and DFL [2,10,11]. When soils become heavier (from loam to clay-loam), fields are placed in central position within toposequences, and groundwater table is shallower (within 50 cm from the soil surface), WUE may rise to 60%.…”

On the effects of winter flooding on the hydrological balance of rice areas in northern Italy

“…where S includes both the variation in the ponding water ( L) and in the soil moisture () within the rice root zone, R is the total rainfall, QIN is the irrigation inflow, QOUT is the irrigation outflow, ETC is the evapotranspiration from soil and/or ponding water and the rice crop, and, finally, SP is a term which includes two main processes: net percolation, namely the net vertical flux at the bottom of the root zone volume (directed downward in flooding conditions), and the net lateral seepage (Bouman et al, 2007;Facchi et al, 2018). In the three pilot areas, since the groundwater level is rather shallow, becoming very shallow (< 1 m) during the flooding periods, the net lateral seepage was assumed to reduce to that through the paddy field bunds.…”