Hydrology’s application to agriculture is found in irrigation, soil conservation and drainage. Agriculture, which for the most part is carried out in natural environments, varies from country to country in its water use. Therefore the way in which hydrology is applied to agriculture will differ according to each country's natural environments.
APPLICATION OF HYDROLOGY TO IRRIGATION
Hydrology helps in Irrigation Planning; it is usually done against such a droughty year as occurs several times over the course of several-ten years, so irrigation water can meet the requirements for most years. Estimation for effective rainfall is essential in irrigation scheduling decisions, is used to design new irrigation systems, and is a guiding factor for planning crop production practices. Estimation of evapotranspiration under various conditions is indispensable to the irrigation planning since evapotranspiration occupies a large part in consumptive use of water on paddy fields as well as on upland fields. Estimation of water resources for irrigation is essential to distinguish if water requirements of farm lands are met by effective rainfall and irrigation water taken from reservoirs and stream flow. The water balance analysis reveals the percentages of precipitation made up by surface flow, evapotranspiration, groundwater recharge and the change of soil storage, all of which are considered useful to the further understanding of the hydrological processes.
APPLICATION OF HYDROLOGY TO DRAINAGE
Hydrology is needed for all applications where drainage capacity is to be computed or analyzed for evaluation or design of drainage improvements. Hydrology is applied to the estimation of Design rainfall and Runoff computation. Design rainfall is defined as the total amount of rain during the cropping season at which or above which the catchment area will provide sufficient runoff to satisfy the crop water requirements. It is usually assigned to a certain probability of occurrence or excess. Runoff computation methods attempt to mathematically reproduce or simulate the hydrologic cycle. They treat rainfall as an input, converting it into estimates of resultant runoff volume and/or rate.
APPLICATION OF HYDROLOGY TO SOIL AND WATER CONSERVATION
Hydrology is applied to the estimation of Soil Conservation Planning and Runoff computation. Determined by the changes in soil water content over time, the soil moisture regime is the main single factor conditioning plant growth and crop production. Comprehensive plan should be worked out on drainage canal network and cropping system in order to maintain a high productivity of farm land. The evaluation of hydrological processes and the selection and development of methods and techniques for correcting or controlling them under different soil, topography and climate conditions are necessary in order to suppress or alleviate the negative effects of soil and water degradation on sustainable agricultural production, water supply in adequate quantities and quality for the different potential uses and on catastrophic events such as flooding, sedimentation, landslides, etc. Analyzing the hydrological approach to assessing and predicting soil and water conservation against processes of degradation would be essential for the adequate development, selection, and application of sustainable and effective use and management practices.