Allocation of scarce water resources using deficit irrigation in rotational systems

On irrigation schemes with rotational irrigation systems in semiarid tropics, the existing rules for water allocation are based on applying a fixed depth of water with every irrigation irrespective of the crops, their growth stages and soils on which these crops are grown. However when water resources are scarce, it is necessary to allocate water optimally to different crops grown in the irrigation scheme taking account of different soils in the command area. Allocating water optimally may lead to applying less water to crops than is needed to obtain the maximum yield. In this paper, a three stage approach is proposed for allocating water from a reservoir optimally based on a deficit irrigation approach, using a simulation-optimization model. The allocation results with a deficit irrigation approach are compared for a single crop (wheat) in an irrigation scheme in India, firstly with full irrigation (irrigation to fill the root zone to field capacity) and secondly with the existing rule. The full irrigation with a small irrigation interval was equivalent to adequate irrigation (no stress to the crop). It is found that practising deficit irrigation enables the irrigated area and the total crop production in the irrigation scheme used for the case study to be increased by about 30- 45% and 20-40%, respectively over the existing rule and by 50% and 45%, respectively over the adequate irrigation. Allocation of resources also varied with soil types.