SIMULHYDRO, a simple tool for predicting water use and water use efficiency in tomato soilless closed-loop cultivations

A simple spreadsheet (SIMULHYDRO) was designed to predict the consumption of both water and fertilisers, and the environmental impact associated to nutrient leaching, in greenhouse soilless cultures on the basis of a limited number of variables (such as global radiation, air temperature, and the ion composition of irrigation water) and parameters, the most important of which is the ion uptake concentration (i.e. the expected ratio between ion and water uptake by the crop). SIMULHYDRO aggregates three major models that run on a daily basis to estimated: i) crop water uptake (VU); ii) the ion composition and the electrical conductivity of recycling nutrient solution (ECNS); iii) the ion composition of drainage water in open (free-drain) and semi-closed (with periodical discharge of the recirculating water) growing systems. SIMULHYDRO was used to simulate the water and mineral relations of greenhouse tomato plants grown in semi-closed substrate (rockwool) culture using saline water (approx. 9.5 mmol L-1 NaCl) and three different fertigation strategies: A) VU was systematically compensated with nutrient solution at full strength (ECNS = 2.5 dS m-1) in order to maintain a (relatively) constant nutrient content; mostly due to the accumulation of ballast ions (Na+ and Cl-), this strategy resulted in a progressive increase of ECNS till it reached a ceiling value (ECNSMAX = 4.5 dS m-1) tolerated by the crop, afterwards the recirculating water was flushed out; B) an EC set-point of approx. 3.0 dS m-1 was maintained and the nutrient solution was flushed out whenever the N-NO3- concentration decreased below 1.0 mmol L-1; C) VU was initially compensated with nutrient solution at full strength, as in Strategy A; when ECNS reached 4.5 dS m-1, the mixing tank was refilled with fresh water only, in order to withdraw N from the nutrient solution before discharge. The experiment included also an open system to verify the possible influence of salinity build-up and/or nutrient depletion on crop yield. A good agreement was found between simulated and measured data. No important effects of the adopted strategies were observed on VU and fruit yield, although the former was significantly higher in open system than in the Strategies A and B. Strategies A and C reduced total water use compared to the others. The results confirmed that a semi-closed system conducted following the strategy of full nutrient solution replenishment (Strategy A) may produce a massive environmental pollution due to nutrient (nitrogen) leaching, although to a much lesser extent than open growing system.