In rice areas with shallow aquifers, an evaluation of alternative irrigation strategies should include the interactions between irrigation and groundwater recharge and reuse, which influence the overall irrigation efficiency. A modelling system composed of three sub-models within a MATLAB framework (a physically based, semi-distributed agro-hydrological model and two empirical models, the former for the channel network percolation and the latter for the groundwater level) was applied to a 1000 ha rice district in the Padana Plain, Italy. The calibrated framework estimates the daily time series of the water supply needed and of the groundwater level for a given irrigation management, based on the inputs provided (agro-meteorology, crop data, soil data, irrigation practices, groundwater table depth upstream of the study area). Five irrigation management strategies, relevant to the area, were compared: (i) wet seeding and continuous flooding (WFL), (ii) wet seeding and alternate wetting and drying (AWD), (iii) dry seeding and delayed flooding (DFL), (iv) dry seeding and fixed-turn irrigation FTI), (v) early dry seeding and delayed flooding (DFLearly). Due to economic advantages, dry-seeded techniques (DFL, FTI) are replacing the traditional WFL in northern Italy. Simulations show that dry seeding leads to a drastic decrease of the water table in April/May, reducing the overall irrigation efficiency of the area, and that DFL (widely adopted in the area) also causes a spike in rice irrigation needs in June when other crops increase their water demand, exposing the area to water scarcity. All the cited management strategies are assessed in the paper and AWD turned out to couple smaller irrigation needs (from June onwards) compared to continuous flooding techniques with a maintenance of the groundwater recharge, especially in the first part of the irrigation season, thus being a recommendable rice management alternative for the study area.

<p>The north-western part of the Padana Plain in Italy is the most important rice district in Europe. Recently, due to an increased frequency of water scarcity periods, the traditional wet seeding and continuous flooding irrigation has been replaced by dry seeding followed by a delayed flooding or by a turned irrigation. Despite the advantages that dry seeding has brought to farmers, this change is leading to unexpected problems, the main of which are: i) the lowering of groundwater levels in the first months of the agricultural season that is reducing groundwater contribution to water discharges in rivers and irrigation networks of the area, limiting the water availability for agricultural areas downstream; ii) a shift to June of the maximum rice irrigation requirement, leading to an exasperated competition between rice and other crops (e.g. maize).</p><p>In the contest of the MEDWATERICE (PRIMA Section2-2018) and RISWAGEST project (Regione Lombardia, RDP 2014-20), an experimental platform was set up in the core of the Italian rice area (Mortara, PV) to compare three rice irrigation strategies in the period 2019-2022: i) wet seeding and traditional flooding (WFL), ii) dry seeding and delayed flooding (DFL) and iii) wet seeding and alternated wetting and drying (AWD). Irrigation water use was monitored and all the other soil water balance components were quantified. At the field scale, irrigation use was found to be in the order: WFL > DFL > AWD, without penalizing rice production, while the temporal distribution of irrigation needs and percolation fluxes (i.e. groundwater recharge) changed as a function of the irrigation strategy.</p><p>Results achieved in the experimental platform were used to set-up a semi-distributed agro-hydrological model simulating water fluxes and storages of a rice irrigation district (about 1000 ha) close to the experimental platform. The modelling framework consists of three sub-models: i) one for the agricultural area, based on the physically-based SWAP (https://www.swap.alterra.nl/); ii) one for the channel network percolation; iii) one for the groundwater level dynamics. Once calibrated, the modelling system was used to explore the effects on the water resources of ‘what-if scenarios’ based on the adoption of specific irrigation strategies in the whole rice-cropped area of the district (about 90% of the agricultural surface) for the period 2013-2020. Besides the aforementioned WFL, DFL and AWD, the following strategies were additionally explored: i) dry seeding and fixed irrigation turns of 8 days (FTI) and ii) early seeding for the DFL irrigation technique (beginning of April). Three indicators were used to support the analysis: i) Water Application Efficiency - WAE, defined as the potential evapotranspiration divided by the irrigation reaching the fields plus rainfall, ii) Distribution Efficiency of the irrigation network - DE, defined as the irrigation reaching the fields divided by the irrigation discharge entering the district, iii) Relative Water Supply - RWS, defined as the irrigation discharge entering the district plus rainfall divided by the potential evapotranspiration. Water fluxes and indicators are calculated and discussed both for the entire agricultural season (April-September) and for the most critical month (June).</p>

<p>The main goals of agro-hydrological models are to estimate water fluxes in the soil-plant-atmosphere continuum (SPAC) and to support management and planning of the water resource over specific areas (a field, a farm, an irrigation district, a river catchment, etc.). Hydrological models are ‘physically-based’ (e.g., SWAP, HYDRUS, FLOWS) if they rely on an accurate mathematical description of processes taking place in the physical system, or are ‘conceptual’ (e.g., SWAT/SWAT+, DSSAT, IDRAGRA) when they are based on a simplified schematization of the physical system and the processes involved. The Richards’ equation (Richards, 1931), modified to include root water extraction, is applied by physically-based models to compute soil water movements, requiring the knowledge of the soil water retention, θ(h), and unsaturated conductivity, K(θ), curves, and the use of quite complex and computationally demanding numerical schemes to be solved. On the contrary, conceptual models are based on a bucket/cascade bucket approach to describe water movement into the soil.</p><p>Conceptual models are often applied for irrigation planning over large areas in a spatially distributed mode, since they require less data and a lower computational effort than physically-based models. However, when considering rice areas, the strong link between irrigation, soil characteristics and groundwater dynamics dictates the need of a rigorous way to compute soil water fluxes, from which the irrigation efficiency depends. This suggests the application of physically-based models or the development of a novel conceptual approach able to describe the effect of compacted soil layers and shallow groundwater conditions on the percolation/capillary rise dynamics.</p><p>This work presents a simplified soil water balance model for rice areas based on the Darcy equation. This approach allows the quantification of vertical fluxes based on a limited number of data that can be retrieved from existent data sources (depth and thickness of the less conductive layer within the soil profile, saturated soil hydraulic conductivity of this layer, ponding water level inside the paddy field and depth of the groundwater table). The new approach was validated comparing the water fluxes obtained with those achieved through a semi-distributed/mechanistic agro-hydrological model based on SWAP (https://www.swap.alterra.nl/) applied to a pilot rice district of about 1.000 ha in northern Italy (San Giorgio di Lomellina, Pavia).</p><p>This research was developed in the context of the MEDWATERICE (PRIMA Section2-2018) and RISWAGEST projects (Regione Lombardia, RDP 2014-20).</p>

<p>Italy is the Europe’s leading rice producer, with over half of total European production. The main rice area is in the north-western part of the country (Lombardy and Piedmont regions). In this area, irrigation of rice has been traditionally carried out by flooding; the introduction of alternative water-saving irrigation strategies could reduce water consumption, but their overall environmental and economic sustainability, as well as their social acceptability, should be investigated.</p><p>An experimental platform was set up in the core of the Italian rice district (Lomellina, PV) to compare different rice irrigation management options: wet seeding and traditional flooding (WFL), dry seeding and delayed flooding (DFL), wet seeding and alternated wetting and drying (AWD). Six plots of about 20 m x 80 m each were set-up, with two replicates for each irrigation option. One out of two replicates for each option was instrumented with: water inflow and outflow meters, set of piezometers, set of tensiometers and water tubes for the irrigation management in the AWD plots. Proper agronomic practices were adopted for the three management options. Periodic measurements of crop biometric parameters (LAI, crop height, crop rooting depth) were performed and rice grain yields and quality (As and Cd in the grain) were determined. Data measured in the field, together with those provided by the farmer, concerning the agronomic inputs and the economic costs incurred for the three irrigation options, were used to assess their economic and environmental sustainability through a set of quantitative indicators. Finally, through interviews with rice growers of the area, barriers to the adoption of the AWD technique were assessed and ways of overcoming them identified. In order to support water management decisions and policies, data collected at the farm level are extrapolated to the irrigation district level through a semi-distributed agro-hydrological model, used to compare the overall irrigation efficiency achieved implementing AWD when compared to WFL.</p>