Green-blue water accounting in a soil water balance

Abstract

It has become common practice to speak about ‘green’ versus ‘blue’ water consumption, in order to distinguish between consumption of rainwater versus groundwater or surface water. The two sources of water differ in terms of possibilities for storage and use. Whereas industrial, municipal and livestock water supply primarily depend on blue water, crop cultivation relies on both green and blue water. Discriminating between green and blue water consumption in a crop field is not straightforward: consumption refers to evapotranspiration (ET) and water contained in the harvested crop, which both appear in undifferentiated form. One cannot see which part of ET or the water in a plant originates from rainwater and which part from irrigation water. In this paper I propose a generic and physically based method to differentiate green and blue evaporation (E) and green and blue transpiration (T) by daily accounting of the fractions green and blue water in each soil and vegetation layer. The green and blue fractions of all water fluxes leaving a soil or vegetation layer in a day depend on the average green and blue water fractions in that soil or vegetation layer during that day. This method allows for an accurate assessment of irrigation efficiency (the ratio of blue water transpiration to the irrigation water applied), and for a precise estimation of green and blue water footprints of crop production (the ratio of either green ET or blue ET to the crop yield).