The Analytical-experimental Approach to Determination of Spacing-depth Relationships of Tube Drains

The current practices used in tube drainage design often lead to unsatisfactory results : from the point of view of the system's hydraulic performance, or from the point of view of high cost of the system in relation to the actual economic benefits derived from it. It is proposed therefore, that the design of tube drainage systems should be based on a synthesis of experimental field results, analysis of hydrological and climatological data, mathematical models of the flow regime within an artificially drained soil. The climate and hydrological uncertainties should be incorporated into the design process, as well as the economic consequences of the various agricultural constraints imposed upon the drainage system. The model to be developed should enable the testing of several spacing-depth combinations for given hydrological input patterns. However, a number of parameters have to be evaluated experimentally. Among these, the entrance resistance of various tube drainage methods should be mentioned; its evaluation under field conditions, using current techniques, is difficult and not without error. Some mathematical solutions of the tube drainage problems (and their experimental verification) have shown that the above mentioned parameters may be lumped together with others and determined experimentally as a "reservoir coefficient", similar to coefficients used in the Theory of Storage. The combination of analytical and experimental methods will serve as a rational basis for the examination of system performance and for the selection of the "optimum design". It will attack drainage problems on a broad front, using a systems engineering approach and applying operations research techniques, combining mathematical flow models, economic criteria, hydrological patterns, and an observed measure of uncertainty of climatic behaviour and crop responses. Where data on crop response to drainage conditions is lacking, the proposed model may indicate the required increase in yield which would economically justify a given design. It is emphasized that such analytical-experimental models should be developed in a number of representative soil profiles found in the problem area. In this way the performance of a given drainage system will be tested under varying soil and land-use conditions.