A methodology for determining irrigation water requirements, Otaki-Te Horo

Abstract

Recent pressure on water resources in New Zealand is forcing planners to take a look at the way in which this finite resource is being used. National total water consumption is estimated to be 2000 million m3 per annum, over 50% of which is used to irrigate New Zealand's extensive agricultural and horticultural development (Department of Statistics 1992). Already much of the country's surface water resources are fully allocated, and underground water resources are under pressure. While demand for water for irrigation is increasing, water resources are beginning to act as a constraint on production and development. Efficient water allocation is therefore essential for future growth of the industry. There is currently little scientific basis behind the allocation of water for irrigation in most regions of the country (Dravid et al 1995). Factors such as soil type, the crop moisture requirements, and the time of the year are not considered. Allocation is often based solely on, the area of land to be irrigated and, a single 'magic' number (usually based on evapotranspiration values) for the entire region, regardless of the variation in soil and crop types, which require varying quantities of water. Since water is essentially free growers have little or no incentive to conserve water and in many cases they are applying more than is required for optimum crop yield. This inevitably leads to an inefficient use of water resources.