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Water harvesting and soil moisture retention - chapter 3,4

Designing water harvesting systems Introduction The water shortage in the cultivated area is supplemented by water from the catchment area (Figure 2). When designing a water harvesting system the size of the catchment area is calculated or estimated, in order to ensure that enough runoff water | 3 Designing water harvesting systems 3.1 Introduction The water shortage in the cultivated area is supplemented by water from the catchment area Figure 2 . When designing a water harvesting system the size of the catchment area is calculated or estimated in order to ensure that enough runoff water is harvested for the crops in the cultivated area. The relation between the two areas is expressed as the C CA ratio the ratio between the catchment area C and the cultivated area CA . For seasonal crops a C CA ratio of 3 1 is often used as a rule of thumb the catchment area C is three times the size of the cultivated area CA. Although calculation of the C CA ratio results in accurate water harvesting systems it is often difficult to calculate the C CA ratio. The data required rainfall runoff and crop water requirements are often not available and if they are variability is often high. They may differ from one location to an other or from year to year. Calculations may give an impression of accuracy but this is misleading if they are based on data with a high variability. For this reason water harvesting systems are often designed using an educated guess for the C CA ratio. Many successful water harvesting systems have been established by starting on a small experimental scale with an estimated C CA ratio. The initial design can then be modified in the light of experience. In order to be able to estimate the C CA ratio and to assess critically the results of the first experimental water harvesting system it is necessary to have a thorough understanding of how water harvesting works. Which aspects influence the functioning of a water harvesting system The following paragraphs will deal with each of these aspects. A formula is presented for calculation of the C CA ratio in the last paragraph. Designing water harvesting systems 13 3.2 The water-soil system The objective of a water harvesting system is to harvest runoff. Runoff is produced in the water-soil system where the .