Optimal Irrigation Quantity and Frequency

Mathematical models are derived for two different irrigation practices: (1) Soil moisture is kept equal or below the optimal soil moisture and (2) soil moisture is allowed to fluctuate above or below the optimal soil moisture. Evaluation of models indicated that the optimal irrigation quantity and frequency can be determined when irrigation costs and average consumptive use are known. The major irrigation cost consists of expense and economic losses. The former is the cost of purchasing and delivering water to the farm and the latter is the loss caused by crops grown under unfavorable soil moisture conditions. The optimal soil moisture, optimal yield and cost functions are obtained from irrigation experiments. Two examples are presented for lettuce and celery crops in Hawaii. The developed technique can be used for other crops if similar information is available. This study showed that the general irrigation practice, irrigating to the field capacity, is not always the optimal irrigation schedule.