A simulation model to generate the demand hydrographs in large-scale irrigation systems

The study reports the development of a model for the generation of daily volumes and hourly discharge hydrographs, withdrawn from on-demand pressurised irrigation systems. The model is based on the simulation of the water budget at the level of each single hydrant. Under the hypothesis that the initial soil moisture is at field capacity, once the soil water reserve falls down a pre-defined limit value, irrigation occurs. The farmer's management strategy was simulated using a stochastic approach allowing for the generation of the initial time of each irrigation at each hydrant. The aggregation of the hydrant hydrographs generates the discharge hydrographs at the upstream end of the network. The calibration of the model was carried out comparing the generated and measured hydrographs at the upstream end of an irrigation network in Southern Italy. The results obtained are satisfactory even though they require further verifications. The comparison has generally shown a good correspondence, particularly for daily withdrawn volumes. The simulated hourly discharges showed, sometimes, hourly peaks higher than the measured ones. The proposed model, when well calibrated, can be used for the design of new irrigation systems as well as for the analysis of existing ones.