Agronomic concepts in water footprint assessment: A case of study in a fertirrigated melon crop under semiarid conditions

This paper compares the assessment of previous methodologies to calculate blue and gray water footprint and includes agronomic concepts that reflect the semiarid scenarios of fertirrigated crops with low water quality. We describe how we have employed these methodologies in a three-year field experiment. The latter involves a fertirrigated melon crop under mineral fertilization, using eleven different N rates ranging from 11 to 393 kg ha−1 N in semiarid conditions, where irrigation is necessary to maintain production. We found that the different methodologies do not consider the scenario where green water footprint is zero, because the effective rainfall is negligible, and the irrigation water has high salt content, requiring the application of larger volumes of water to avoid salt accumulation in the soil and consequent loss yield. We propose modifications to the calculation of water footprint to consider this scenario. In our calculation the blue water footprint includes: (i) the extra consumption of irrigation water that the farmer has to apply to compensate the lack of uniformity in drips discharge; (ii) the water requirement to consider percolation losses and salts leaching, which depends on the salt tolerance of the crop, soil and quality of irrigation water, needed to ensure the fruit yield. With respect to gray water footprint, all N sources susceptible to being lost were considered, the N fertilizer rate and N content in the irrigation water and in the soil (mineral N and mineralized N during the crop period). Therefore, besides considering all these parameters, our proposal takes into account the drained water, given with a water balance, and the nitrates amount below the roots and susceptible to being washed. The methodologies of previous studies underestimate the water footprint resulted in our experiment. With the new considerations proposed, the treatments with the optimum N dose obtained a total water footprint between 127.8 and 151.7 m3 t−1. Higher values than those were presented in the treatments with the least N dose (145.7 and 158.4 m3 t−1), although the highest values of water footprint were obtained in treatments with a N excess (226.0 and 355.0 m3 t−1).