Evaluation of soil moisture tension for starting irrigation applying decrease of consumptive use with soil dry on paddy based land in tropical area: study on diversified crop water requirements in the Philippines (IV)

摘要

This study presents an evaluating method of soil moisture tension (pF) prior to irrigation for obtaining maximum yields. Seven crops are grown under three different points of starting irrigation, which are the initial wilting, medium and optimum points. Ratios of consumptive use (CU) to pan evaporation (Epan) per day. are computed for test spots obtaining maximum yields under the three. CU/Epan ratios decrease with increase of pF values, each of which is the last pF of 24 hours for determining CU per day Two patterns appear on the decrease of CU/Epan. One pattern is that decrease is gentle up to a marginal pF, and then sharp. Four crops show this pattern. The marginal pF values of four crops are verified and recommended for the pF values of starting irrigation to obtain maximum yields, by applying the relationships of CU with yields and real data. Another pattern is that decrease is gradual up to a marginal pF, and then little. Three crops show this pattern which show small yield reduction under the dry. The marginal pF values of three crops are too low to be applied It is therefore recommended that pF values of starting irrigation are decided in considering availability of labor and water resource.