Assessment of NSWRM effectiveness under current and future climate at the field and farm scale. Deliverable D4.3 of the EU Horizon 2020 project OPTAIN

Deliverable report D4.3 of the EU Horizon 2020 Project OPTAIN (Grant agreement No. 862756) The main objective of this deliverable D4.3 is to perform an integrated, model-based assessment of the effectiveness of Natural/Small Water Retention Measures (NSWRMs) under current and future climate conditions at field scale. The assessment is based on the adaptation of the SWAP field-scale mathematical model to seven pilot sites across three European biogeographical regions and its cross-validation with the catchment-scale SWAT+ model, used in the OPTAIN project. This report contains a detailed description of the SWAP modelling workflow, from input data preparation, model setup and harmonisation, model calibration and application in climate and NSWRM scenario runs. It provides calibration and scenario analyses results from seven case studies, out of which four groups joined the field-scale modelling work voluntarily. In the results section, for each of the three biogeographical regions we i) give an assessment of the effectiveness of NSWRMs with respect to water retention at present and future climate conditions and 2) compare the SWAP and SWAT+ simulation results for the pilot fields and selected water balance elements. We concluded that the NSWRMs can contribute to water retention within the landscape. While this effect appears to increase for land use change related measures in the future, it appears to decrease for management-related measures. The cross-validation of the water balance elements of the two models showed that the SWAP and SWAT+ simulation results were comparable for the status quo (present situation, for which the models have been calibrated), but differed for the NSWRMs scenarios, depending on how the measures were implemented in each model. Our SWAP modelling results on the effectiveness of measures were, in most of the cases, comparable with the experimental data from the WOCAT database. This analysis, however, revealed that there is a strong need for region-specific reports on the effects of NSWRMs on soil water balance elements for improving the SWAP simulation results of the NSWRMs effectiveness in retaining water within the soil profile and the landscape. Summary Within the OPTAIN project, the effects of Natural/Small Water Retention Measures (NSWRMs) on water regime, soil erosion and nutrient transport are evaluated at both catchment- and field-scales for present and future climate conditions. The goal of this deliverable report D4.3 is to perform an integrated, model-based assessment of the effectiveness of NSWRMs at the field scale and to use these results for cross-validating the outputs obtained from the catchment-scale modelling. The assessment is based on the adaptation of a field-scale mathematical model (SWAP) to seven pilot sites across three European biogeographical regions and on combined analyses of NSWRM and projected climate scenarios. The scenarios are designed to evaluate the efficiency and potential of different NSWRMs in improving soil water retention and reducing flash floods and the loss of soil and nutrients under changing climate conditions. This report contains a detailed description of the SWAP modelling workflow, from input data preparation, model setup and harmonisation, model calibration and application in climate and NSWRM scenario runs. It presents calibration and NSWRM scenario results from seven OPTAIN case studies from three different biogeographical regions (Boreal, Continental and Pannonia). The report also describes i) the new approaches and tools developed within the OPTAIN project that facilitate the implementation of the scenarios and the interpretation of the modelling results, ii) the methods used to cross-validate the SWAP and SWAT+ models, and iii) the issues faced during the implementation of this work. The SWAP model was calibrated for all the pilot fields with good or satisfactory results. The impact of four in-field NSWRMs - reduced tillage, shifting to grassland, afforestation and drought tolerant crops - on the water balance elements was evaluated. The scenario results indicate that the effects of measures on soil water retention and other water balance elements have some regional pattern, but can be strongly dependent on local conditions (e.g. soil, crop, slope). According to the scenario results, for most of the cases the studied NSWRMs contributed to reducing evaporation, surface and subsurface runoff and percolation to deeper layers, which results in increased soil water retention or plant water uptake within the fields. The cross-validation of the field-scale SWAP and catchment-scale SWAT+ models was a challenging task and could only be performed for selected water balance elements (evaporation, transpiration and drainage outflow). Comparable results were obtained in most of the cases for the baseline scenario, but the differences between the soil water balance elements simulated by the two models increased when implementing the different measures. The increased differences, however, could also reflect the differences in measure implementation, as these were constrained by the model’s structure and parameters. We concluded that the implementation of the SWAP field-scale model in the scenario analysis and cross-validation could positively contribute to i) better understanding the effects of NSWRMs at field level and ii) evaluating the outputs of the SWAP and SWAT+ models in a wider context. We concluded that NSWRMs can contribute to water retention within the landscape, and that this effect seems to decrease and increase in the future for measures related to management and land use change, respectively. The cross-validation of the water balance elements of the two models showed that the SWAP and SWAT+ simulation results were comparable for the status quo (present situation, for which the models have been calibrated), but differed for the NSWRMs scenarios, depending on how the measures were implemented in the two models.