Water management in irrigated rice - Rice Knowledge Bank ...

Fig. A.2. Groundwatertube for monitoringgroundwater depth belowrice fields.Cover(used tin can)5-cm-diameter PVCpipeBund surface50 cmReadhere100 cm 200 cmMeasuringdeviceGroundwater levelHoles are 5 mmin diameter, 2 cm apart50 cma hollow and bottomless PVC-tube of about 20 cmin diameter and 35 cm long with small holes using adrill. The holes should be about 0.5 cm in diameterand spaced 2 cm apart. Install this tube in the fieldso that the bottom of the tube is buried in the plowsole (about 20 cm deep) and that some 15 cm ofthe tube protrudes above the soil surface. Removethe soil from inside the tube down to the bottom ofthe tube. Water will flow through the holes into thetube, so that the water level inside the tube is thesame as outside. After irrigation, the level of thewater in the tube can be seen going 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 into the field). Make sure that thespot chosen to install the tube is representative forthe whole field (don’t place it in a depression or inan elevated patch).The water depth is measured from the top ofthe tube to the level of the water in the field usinga simple ruler. Subtract 15 cm from the reading toobtain the depth of ponded water. A negative valuemeans that the water is standing on the field; a positivevalue means that the water level is below thesurface. To make the measurement more accurate,the height of the tube protruding above the surfacecan be measured a few times during the season tocheck whether it is 15 cm.A.2 Groundwater tubeThe groundwater tube is used to measure the depthof the groundwater below the rice fields (Fig. A.2).Cut a length of 175 to 250 cm of hollow and bottomlessPVC-tube of about 5 cm in diameter. Use adrill to make holes along a section of 50-cm lengthat one end of the tube. The holes should be about46