Water management in irrigated rice - Rice Knowledge Bank ...
Water management in irrigated rice - Rice Knowledge Bank ...
Water management in irrigated rice - Rice Knowledge Bank ...
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Fig. A.2. Groundwatertube for monitor<strong>in</strong>ggroundwater depth below<strong>rice</strong> fields.Cover(used t<strong>in</strong> can)5-cm-diameter PVCpipeBund surface50 cmReadhere100 cm 200 cmMeasur<strong>in</strong>gdeviceGroundwater levelHoles are 5 mm<strong>in</strong> diameter, 2 cm apart50 cma hollow and bottomless PVC-tube of about 20 cm<strong>in</strong> diameter and 35 cm long with small holes us<strong>in</strong>g adrill. The holes should be about 0.5 cm <strong>in</strong> diameterand spaced 2 cm apart. Install this tube <strong>in</strong> the fieldso that the bottom of the tube is buried <strong>in</strong> the plowsole (about 20 cm deep) and that some 15 cm ofthe tube protrudes above the soil surface. Removethe soil from <strong>in</strong>side the tube down to the bottom ofthe tube. <strong>Water</strong> will flow through the holes <strong>in</strong>to thetube, so that the water level <strong>in</strong>side the tube is thesame as outside. After irrigation, the level of thewater <strong>in</strong> the tube can be seen go<strong>in</strong>g down every day.The tube can be placed at the side of the field closeto the bund (but at least 1 m away from the bund),so it is easy to record the water depth (no need towalk very deep <strong>in</strong>to the field). Make sure that thespot chosen to <strong>in</strong>stall the tube is representative forthe whole field (don’t place it <strong>in</strong> a depression or <strong>in</strong>an elevated patch).The water depth is measured from the top ofthe tube to the level of the water <strong>in</strong> the field us<strong>in</strong>ga simple ruler. Subtract 15 cm from the read<strong>in</strong>g toobta<strong>in</strong> the depth of ponded water. A negative valuemeans that the water is stand<strong>in</strong>g on the field; a positivevalue means that the water level is below thesurface. To make the measurement more accurate,the height of the tube protrud<strong>in</strong>g above the surfacecan be measured a few times dur<strong>in</strong>g the season tocheck whether it is 15 cm.A.2 Groundwater tubeThe groundwater tube is used to measure the depthof the groundwater below the <strong>rice</strong> fields (Fig. A.2).Cut a length of 175 to 250 cm of hollow and bottomlessPVC-tube of about 5 cm <strong>in</strong> diameter. Use adrill to make holes along a section of 50-cm lengthat one end of the tube. The holes should be about46