Study On Water Distribution Uniformity Of Sprinkler System

Water distribution uniformity coefficient of sprinkler irrigation is a strategic index for evaluating sprinkler systems, and usually it is shown as Christiansen Coefficient Uniformity—CU. In the design stage, it is difficult to measure the CU under different combinations of sprinklers (different layout of sprinklers, space of sprinkler and space of laterals) by field experiments. So the method of overlapping is usually used to get CU under different sprinkler combinations based on the data of single sprinkler water distribution. There are two layout formats of rain catchers, radial layout and grid layout when measuring water distribution of single sprinkler in field. The workload of radial layout is far less than that of grid layout. So in practice the radial layout with eight radial or twelve radial is often taken.The single sprinkler water distribution data of radial format could not be used to compute CU directly. A method named "interpolating radial first, then interpolating along arc direction" is put forward to transform the data in radial format into data in grid format so that the matrix could be used to compute CU by computer. A computer programme is designed to complete the transformation and enhance work efficiency.After the computation of CU of rectangular combination of sprinkler under diversified spaces of sprinklers and laterals is discussed, the concept of "equivalent area" is raised to solve the difficulty in computing CU of triangular combination. In the concept of "equivalent area" the typical area of overlapping of triangular shape formerly is reset to an area of rectangular shape. The latter area has coequal representative as the former area. Thus, in the actual design of sprinkler systems, when the type of sprinkler is determined, the CU under diversified combinations with different spaces of sprinklers and laterals can be obtained just by using the water distribution data of the specified sprinkler with corresponding work pressure. This helps to compare different designs and determine an optimum one that make the CU satisfy the prescript of the state criterion as well as lower the system investment and operating cost.Davis(1963), Hart(1972) and Li J(1998, 2002) found that water distribution uniformity subsurface is higher than that the water applied on surface after period of time of infiltration. The crop yield mainly responses to the water distribution in the root zone, so from the standpoint of the high yield of crop that investigation in water distribution within the soil is more important than the areal water distribution.The unsaturated soil water movement equation based on the Darcy's Law and the principles of quality conservancy is the base of investigation on unsaturated soil water flow. Much research on unsaturated soil water movement has been carried out together with different boundary conditions and initial conditions. Sprinkler irrigated water infiltration is a kind of free infiltration of unsaturated soil. Certainly, the sprinkling intensity should always be less than soil permitted intensity. The paper determined the investigated range of soil water movement with sprinkler irrigation based on the typical overlapping area and the corresponding initial condition and boundary conditions, and then design computer programme to solve the basic equation by using economical arithmetic—a three dimensional implicit alternating-direction finite-difference method(advanced by Brian).In order to validate the proposed mathematical model and the corresponding solving method and accumulate basic experiment data for soil water movement and water distribution within sprinkler irrigated fields, two experimentations were progressed in the Comprehensive Irrigation and Drainage Experimental Station of Wuhan University. The results of the two experimentations show that: though the value of CU is just about 76%, the soil moisture becomes well-proportioned in 16~24h after sprinkling. The uniformity coefficient of soil moisture can reach 90% and higher. The parameters of soil water movement(unsaturated soil hydraulic conductivity K and diffusivity D) are simulated based on the field measured characteristic curve of soil moisture by using van Genuchten model. Then the experiment is simulated by using the designed computer programme.Also the variety progress of the uniformity of the soil moisture with different sprinkling conditions is simulated. Factors such as CU, total applied water, initial soil moisture and soil properties etc are involved. Some conclusions can be drawn, and listed blow: (1) Case with high initial soil moisture provides rapid soil water movement. And after the fall of uniformity of soil moisture, the value of uniformity coefficient of soil moisture raises at a relatively high rate towards the maximumdistribution. That is to say that the whole variety progress of high initial soil moisture completes earlier than that of low initial soil moisture. But the soil moisture of both cases reached a high plateau in considered period. (2) uniformity coefficient of soil moisture of the case with high value of CU varies less than that with high value of CU. (3) in the same time period, the case with high application rate provides rapid soil water movement, and during the water movement progress the least uniformity coefficient of soil moisture of the case with high application rate is less than that with lower application rate. But the low uniformity coefficient of soil moisture rises at a higher rate soon and reaches the high plateau first. In spite of which factor is considered, there is a ubiquitous characteristic in all cases, i.e. the uniformity coefficient of soil moisture can reach a high value in considerable time period after irrigation.