SEEPAGE ANALYSIS FROM FURROW CHANNELS

Abstract

Furrow irrigation is a conventional method of irrigation used in tropical region in India to grow vegetable crops. Many researchers have reported that the furrow irrigation has lower application efficiency in comparison to drip irrigation. The obvious reasons for lower efficiency is due to seepage loss from the furrow when irrigation water is conveyed in the furrow for a time more than required besides due to inappropriate spacing of the furrows. In this thesis, the spacing of the furrows has been specified so that 75% of the root zone remains within the phreatic surfaces. While ascertaining the spacing, the soil capillarity has been accounted. The required duration of water supply in the furrows has been ascertained, equating the arrival time of a soil moisture saturation front at the centre of the furrow ridge in a horizontal capillary, to the arrival time of a soil moisture saturation front in a vertical capillary beneath the bed of the furrow at a depth, such that the excess soil moisture above field capacity within the saturation front in the vertical capillary is equal to the soil moisture required to bring the moisture in the remaining part of the root zone to field capacity. Water should be conveyed in the furrow up to this time so as to avoid unnecessary percolation losses and seepage losses. In order to ascertain the performance of furrow channels, and design the furrow spacing based on steady state flow condition, the following seepage problems have been solved using inverse hodograph method and Schwarz Christoffel conformal mapping technique: I) Seepage from a triangular furrow considering capillarity. 2) Seepage from parallel triangular furrows. Seepage from an array of triangular furrows considering capillarity. The symmetry in the flow domains has been used for reducing the number of vertices in the mapping function. The slope of the tangent to the phreatic line, —dy cbc attains the maximum value at the point of inflection. For the slope of the tangent to the phreatic line at the point of inflection to be the maximum, real part of —dz at the dw point of inflection i.e. is the maximum. This condition has been used for (u +v2) finding the transformation parameters while solving seepage from an array of triangular furrows in soil possessing capillarity. The improper integrals appearing in the mapping procedure have been evaluated using Gaussian weights and abscissa after removing the singularity. One of the advantages of using Gaussian abscissa is that the limits of the integrals are approached and not exactly included for computing the integrand. The boundary condition assumed in the analysis is Neumann-type i.e. at infinity the hydraulic gradient is 1. This boundary condition implies that the seepage from one of the furrows is hydraulic conductivity times spacing of the furrows irrespective of the shape of the furrows. Thus the problem of computing seepage is very much simplified by assuming Neumann type boundary condition to prevail at infinity. Based on the study the following conclusions have been drawn: The seepage, Q, from a triangular furrow varies quasi linearly with , capillary suction head, --- . The capillary suction head causes more seepage loss from • a furrow. The capillary rise ordinate, Ys at the bank of the furrow is linearly ys he proportional to the capillary suction head—he . For any bank slope (m > 0), < H H If the dimensionless ridge height above water surface is less than the ordinate In- , the ridge will be saturated up to the ground surface. In that case, the phreatic surface • instead of starting at the inclined face of the furrow will start at the horizontal ground surface. On the basis of a criterion, that is, for any spacing and side slope of the triangular furrows, the maximum depth to phreatic surface from ground level at the middle of the furrow ridge, (yE + fb ), should be less than one fourth of the root zone depth, furrow spacing has been examined for a free board fb = 5 cm, depth of water in the furrow, H = 10 cm. For deep rooted crops, a spacing of 36cm and m=1 satisfy the above criterion. The locus of the phreatic line is governed by the soil capillarity of the porous medium as well as by the furrow spacing. The magnitude of capillary rise in the furrow ridge increases with decreasing furrow spacing and tends to suction head at the closest furrow spacing. Using Green and Ampt theory, soil moisture movement from a furrow having curvilinear boundary has been analyzed. Based on the movement of the saturation front in root zone, the spacing of the furrows and duration of irrigation water supply in the furrows have been predicted for soil having low hydraulic conductivity and high capillarity. The limiting value of furrow spacing should be ascertained using steady state flow condition. The duration of irrigation supply should be decided on consideration of unsteady state flow condition. The scope for further study in respect of seepage analysis has been suggested.