Infiltration characteristics of furrow irrigation in a heavy-textured soil

The objective of this thesis is to study the rate and pattern of infiltration of soil water, under the conditions of heavy texture and shallow depth in a tropical furrow- irrigated soil. The analysis is the result of a series of field-experiments and is supported by theories that has been proposed by others.The experiments were carried out in the Cojedes-Sarare Irrigation Project, Portuguesa State, Venezuela. Furrows with a length of 200 m, spaced at w = 0.70 m, and with an average slope of 0. 18 % were used. Three series of experiments were set out: (i) First series with variable inflow and surface roughness; (ii) Second series with variable initial soil moisture content; (iii) Third series with variable furrow length. Replicates of the treatments were distributed at random.Five irrigations were applied to the land during the period from January to March, 1970. Subsequently in the first series of experiments, first, third and fourth irrigations for three roughness conditions and four sizes of flow were tested. The second irrigation was used for the second series of experiments. The fifth irrigation served for the third series of experiments.During the first series of experiments, the following measurements were taken: (i) rate of advance of the water front (distance x in m at time t in min); (ii) furrow section parameters (top width T and depth h ); (iii) furrow inflow Q and outflow Q _out . During the second and the third series of experiments, only the simultaneous inflow and outflow were recorded.Advance and infiltration functions were obtained for the period of advance of the water front (first stage), and infiltration functions for the period of wetting the root zone (second stage). Exponential equations were obtained by computer analysis for single furrow trials. Then, by averaging coefficients and exponents of the equations of the replicates, general equations for each treatment were found.The data of x as a function of t showed a good fit with the equation x = p t ^r . The coefficient p increased significantly with the flow size Q and the exponent r showed a trend to decrease although not significantly, with increasing Q . The coefficients of variation of p and r were rather high. Therefore a single furrow advance trial may not suffice to express the average field advance of the water front under the given conditions.The advance curves showed that the differences in roughness were great between the first irrigation with loose furrows and those irrigations after two or three applications have taken place. The roughness conditions appeared to be identical for third and fourth irrigations.With distance-averages of the furrow section parameters h and T , for three water front advance stages ( x = 87.5 m, x = 137,5 m and x = 175.0 m), the average section a _f , and the average wetted perimeter P were obtained for a parabolic section of the furrows. The surface volume V _f = a _f  p t ^r , and the area of infiltration A _i (net area A _in = P pt ^r and gross area A _ig = w pt ^r ) were then arrived at.The infiltration functions were found for each treatment during the first stage, as V _i= f(t) by using single furrow data of V _i = Q t - V _s ,. As the average infiltration depth I _cum = V _i / A _i , the equations for I _cum= f(t) were obtained. Equating these functions with the equation I _cum = F at ^b+1/( b + 1) ( b + 2), the parameters a and b of the Kostiakov equation ( I = a t ^b ) were derived. For the second stage (when x = L = 175.0 m), the infiltration function was obtained by simultaneous measurements of the inflow and outflow, as infiltration flow: Q _i = Q - Q _out , from which the parameters of the infiltration equations, were found.The increase of infiltration with inflow size was clearly shown from the data analysis of both stages as being the effect of a larger volume of water. The parameters of the infiltration equation for the first stage altered in successive irrigations.Some emphasis was put on the unit inflow function q0 to relate flow sizes for both stages with length of run and infiltration. Equations for the unit inflow q₀ = Q / A _i and for unit infiltration flow q _i = Q _i / A _i per unit area, were obtained for each treatment. Then a generalized type of equation was introduced which relates the unit inflow function with the average depth of water infiltrated during the advance time at the furrow intake. An equation to predict the length of advance is included x = φ( Q ) t ^0.927, for the surface roughness and soil conditions under which the experiments were carried out. The representation of q₀= f(t) and q _i = f(t) for both stages, in a composite figure with the advance function as a function of time, provides an illustration of the infiltration process, usable for the design and management of furrow irrigation under the conditions of the experiments.The relationship between the exponent of time in the advance equation and the exponent of time in the infiltration equation was analysed with the data from the experiments. This analysis confirmed that r increases when ( b + 1) decreases. This agrees with findings in the literature, such as the relationship proposed by FOK and BISHOP (1965) Values for the surface storage coefficient C ₁ = D / D₀ , and infiltration coefficient C ₂ = I _cum / I _cum0 to solve the balance equation for predicting advance were also obtained.The second series of experiments, in which infiltration rate was measured during the second stage, as a function of the initial moisture content, showed that the value of the coefficient a of the Kostiakov equation increased not significantly as the initial content of soil moisture decreases.The third series of experiments - measurements taken during the second stage - showed that upon the increase of furrow length, the coefficient a of the infiltration equation decreases and the exponent b increases.Water losses by deep percolation and by run-off at the end of the run, were finally analysed on the bases of the equations found and the data available. The analysis was made for the case of constant inflow for both stages (third irrigation), and for the case of reduced inflow during the second stage (fourth irrigation).The data analysis showed that infiltration is a very variable factor affected by the conditions of the soil and the surface of the channel bed, as well as by the size of the flow, furrow length and stage of irrigation. Soil cracking upon drying was found to be a relevant factor in the entry of water into the soil. Because deep percolation losses are certainly very small under the indicated physical conditions, irrigation efficiency will be rather high if provisions are made to use a cut-back stream, during the second stage, in order to lose a minimum of water by run-off at the end of the run.