Spatial and temporal estimation of the green and blue Remote Sensing-based Agriculture Water Accounting and Footprint at the Pinios River Basin

Indicators on the sustainability of productive human sectors help boosting societal awareness and provide remarkable information for political decision-making and resources management. Prominent examples of relevant environmental indicators currently available are those that form the footprint family. In agriculture, the water footprint approach provides indicators that integrate direct and indirect freshwater usage. While a considerable number of studies developed so far used tabulated values for crop parametrization, the less explored application of dense remote sensing time series provides huge benefits.

This paper aims to present the spatiotemporal estimation of the green and blue Remote Sensing-based Agriculture Water Footprint (RS-AWAF) at the Pinios River Basin (11,000 km²) in Greece (year 2017), combining two globally accepted and operational methodologies: the Soil Water Balance published by the Food and Agriculture Organization in its irrigation and drainage paper 56 for water accounting purposes, and the standardized methodology for Agricultural Water Footprint estimation of growing a crop or tree published by the Water Footprint Network. Initially, the RS-AWAF applies dense temporal series of the Normalized Difference Vegetation Index produced by Sentinel-2 data at 10m spatial resolution to monitor the crops provided by local authorities through the Land Parcel Information System and derive the biophysical parameters along its development, such as the basal crop coefficient and the fraction of soil surface covered by vegetation. Those are then integrated into a validated and operational Remote Sensing-based Soil Water Balance that day after day and within a pixel spatial scale, estimates among other components of the balance, the adjusted crop evapotranspiration (ET_cadj) and the net irrigation requirements (NIR). In a second step, both previous components are combined to estimate the blue crop water use (CWU_blue), related to the NIR, and the green crop water use (CWU_green), related to the fraction of the ET_cadj that comes from other freshwater sources different than irrigation, the precipitation. Finally, crop yield values collected from official statistics per crop or crop group are used to estimate the blue water footprint (WF_blue) and the green water footprint (WF_green).

Once the green and blue RS-AWAF is estimated, a collection of thematic maps over the Pinios River Basin is ready for use by local stakeholders at their desired working scale. In that sense, monthly and annual thematic maps of ET_cadj, NIR, CWU_green and CWU_blue are available, as well as annual thematic maps of WF_blue and WF_green. In parallel, tabulated values are created from these parameters using zonal statistics through GIS at the spatial scale appropriate to the final user (i.e. water user associations).

These results are part of the EU Horizon 2020 project REXUS (Managing Resilient Nexus Systems Through Participatory Systems Dynamics Modelling), in which stakeholders from water user associations to river basin water managers are evaluating the information. At this stage, our final goal is to provide spatiotemporal distributed accounting of agricultural freshwater resources over large areas that enhance regional knowledge and increases efficiency in water management and subsequently contributing to energy-saving, since the major agricultural water volume is abstracted from deep groundwater wells.