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Hydraulics Head Loss Near Agricultural Drains

18 January 2010 12,027 views No Comment

By Robiyanto Hendro Susanto (Doctor Of Philosophy, North Carolina State University, 1993) under the directions of R. W. Skaggs and R. L. Huffman.


The effectiveness of a drainage-subirrigation system is dependent on hydraulic head losses near the drain. The objectives of this study were to quantify hydraulic head losses for agricultural drains under both drainage and subirrigation; to determine the effective radius of the drain and the hydraulic conductivity in the immediate vicinity of the drain; and to investigate the effect of various types of media near the drain on drainage and subirrigation performance. Head losses near the drain, and their effects on flowrate where observed and/or calculated in a 14-ha drainage subirrigation experiment. Hydraulic heads near the drain were measured using piezometers installed inside the drain, and at distances of 10 cm, 20 cm, and 30 cm above the center of drain. Water table data were observed from wells installed on the top of the drain, and at distances of 0.3 m, 0.6 m, 5.7 m, and 11.4 m to the sides of the drain. Drain flows were measured both manually and automatically at the time of the piezometer and well readings. Measurements were conducted in five experimental units having conventional drainage, controlled drainage, and subirrigation.

Large head losses occurred near the drain. The head losses in the vicinity of the drain were typically equal or greater than the head loss in the profile from the mid-plane between the drains to the drain. The major reason for the high head losses was that the values of K near the drain were very much less than the lateral K in the soil profile. In some cases there was additional high head loss across the drain tube wall, but this did not occur in all plots. With one exception, effective radius, re , was equal to or greater than the actual drain radius for controlled drainage and subirrigation. However, it was much less than the actual radius for conventional drainage. Likewise, head losses across the drain tube wall were relatively small for controlled drainage and subirrigation, but ranged from  25 to 34 cm for conventional drainage.

A laboratory study was conducted to: quantify hydraulic conductivity in the immediate vicinity of agricultural drains used on both drainage and subirrigations; evaluate uniformity of K near the drain; and determine the effect of various media near the drain of performance during drainage and subirrigation.

A large undisturbed core with a section of the drain in the center was collected from the field experiment. Disturbed soil around the drain was also taken, dried, sieved, and repacked for a separate packed core experiment. The field core and packed sand or soil cores were placed inside tanks to subject the cores to radial flow conditions stimulating conventional drainage and subirrigation. The drain tube was divided into four sections so that flow from sets of tensiometers, one for each section of the drain. Tensiometers were placed along radial transects at 8.0 cm, 12.0 cm, 16.0 cm, and 23.0 cm from the center of the drain in the field core. Inside the packed cores, tensiometers were installed at distances of 6.5 cm, 9.5 cm, 12.5 cm, and 16.0 cm from the drain center. Two complete sets of tensiometers were installed, separated by a distance of 10.0 cm along the drain. One replicate was connected with 2 mm diameter clear polyethylene tubing to a manometer board. The other replicate was connected to a rotary valve connected to a pressure transducer. Flow rates for each of the drain sections were measured manually directly after the manometer readings were taken.

Flow into the drain decreased during drainage experiments even though the water in the tank was held constant at a fixed level above the center of the drains in the field and packed cores. When the drainage event was discontinued and then started again, the drainage rate recovered to a value greater than that prior to the end of the previous drainage experiment for the field core. Drainage rates were not increased in either of the packed cores. Flow recovery also occurred after subirrigating the core. Flow rates were substantially increased when the flow gradient was reversed and the core placed  under subirrigation boundary conditions. The subirrigation rates were for controlled drainage were much higher than during the previous conventional drainage experiment. The flow rates decreased with time but remained higher than that during previous conventional drainage experiments.

Most of the flow entered the bottom section of the drain during conventional drainage experiments for the field core. when one of the four sections was blocked, flow increased immediately in the other sections such that the total drainage rate was not affected, or the effect was very small. Total flow remained the same and there was no change in the piezometer readings. This indicates that, even if the flow is radial as assumed, the water may move around the drain (within the corrugation) to enter at the place of least resistance.

Average saturated hydraulic conductivity of the field core was obtained by assuming radial flow and using the measured total flow rates and the boundary conditions inside and outside the cores. Saturated hydraulic conductivity, K, decreased with time more rapidly than the flow rate. The change in K between experiments was also greater than the change observed for flow rates. These differences may have been caused by violation of the radial flow assumptions. Another sources of error may have been time lags n the tensiometer readings. Hydraulic conductivity values measured for the soil near the drain for the field core were much higher than values determined in the field. Values of K were increased after subirrigation and were higher for controlled drainage where the drain tube was full of water. Results similar to those for the field core were obtained for the packed sand core and repacked soil core.

Results of K determined from flow rates into each of four drain sections and the corresponding hydraulic head measurements indicated that K in the bottom section was normally higher than in the other section caused flow to move around the drain and contribute to flow in another section. Due to this fact, one can not conclude whether or not the K in the bottom section of the drain was higher or lower than the K in any other sections, nor whether there was uniformity of K near the drain.

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