The Application of Relative Humidity Index to Agricultural Drought Monitoring

Relative humidity index and corresponding methods are introduced to deal with daily agricultural drought monitoring. The method recommended by FAO is used to calculate the potential evapotranspiration and rectify the crop coefficient.When rectifying the crop coefficient under soil water stress condition, the absorbable total available soil water in the root zone is used to replace the total available soil water in the root zone. Because the latter causes higher results, which means water stress seldom occurs. Actually, most of the roots are located in shallow layer, only a small part of major roots extends to deep layer. For example, the roots of winter wheat can be 1.0 m deep before winter, but 80%—85% of the roots are in the soil shallower than 0.4 m. Therefore, if water stress occurs in the upper soil, the winter wheat's growing will be limited despite enough water in the deeper soil, because 15%—20% of the roots in deeper soil can't absorb enough water for transpiration. So it's necessary to consider the water absorbing capacity of the crop root, that's why the absorbable total available soil water in the root zone is used.When calculating the relative humidity index, steady rule can't be reflected reasonably in any individual period, because the former field water balance affects the current soil water condition. By summing up the weighted relative humidity index during different period of time, the combined relative humidity index is evolved, and it can well reflect the cumulative affection of former field water balance.The relative humidity index of single crop reflects the soil water condition on one kind of crop, regional combined relative humidity index is evolved by summing up the combined relative humidity index of single crop weighted on its planting area. With this method, agricultural drought monitoring can step over single crop growing season, so and agricultural drought can be monitored throughout the whole year.There are 114 stations for soil moisture manual observation in Shandong Province. From 28 Feb 2008 to 8 Dec 2008, manual observation is carried out every ten days, and 3048 groups of data are available. The comparison shows that for 2012 groups of data, drought degree calculated from relative humidity index is coincident with observation, reaching 66%. The accuracy is lower than 50% in most of west-north area of Shandong, the irrigation district of the Yellow River, the Dongping Lake, and the Weishan Lake. On the other hand, the accuracy is mostly greater than 70% in east-central of Shandong and some other districts. Among the whole period, the degree of coincident is lower in spring than that in summer.