How does a shelter of solar panels influence water flows in a soil-crop system?

Associating on the same land area an upper layer of solar panels together with a crop layer at the ground level has been shown to allow significant saving of land resource compared to separate energy and food productions (Marrou et al., 2013a). Indeed, crops can achieve high yield under the fluctuating shade of these agrivoltaic systems. Moreover, under dry Mediterranean climate, microclimate measurements at crop level below these panels suggest that these systems could contribute to alleviate climatic stress and to save water. On two experimental prototypes of these agrivoltaic systems, we combined two complementary approaches to assess the impact of the solar panels cover on crop water use. First we calculated the bulk actual evapotranspiration (AET) of irrigated lettuces and cucumbers grown in agrivoltaic systems and in the full sun, from field measurements using the water balance equation for a crop-soil system. Then, we proposed a conceptual framework to analyze AET modifications in the partial shade and assess the contribution of identified drivers to this change. This conceptual framework breaks AET into two components (plant transpiration and soil evaporation) and four drivers: climatic demand at canopy level (ETO), fraction of radiation intercepted by the vegetation, plant stomatal conductance, and soil surface hydraulic conductance. From specific field measurements, we assessed the contribution of each driver to the variations of evapotranspiration below the photovoltaic panels (PVP), in comparison with the full sun. Crop AET calculated with the first approach (water balance) was reduced in agrivoltaic systems by 10-30% when available light was equal to 50-70% of full sun radiation, with variations according to the weather season. The second approach showed that reduction of evapotranspiration was mainly driven by the reduction of the climatic demand below the solar panels and did not result systematically in an increase of the water use efficiency, depending on the genotypic plant sensibility of dry matter accumulation to shade. The conceptual framework suggest that water use efficiency in agrivoltaic systems could be increased by selecting crop species and varieties with a rapid soil covering, which contributes to increased light capture and to decreased soil evaporation, leaving more water for plant transpiration and thereby for biomass production