Rice Field Hydrology

January 15, 2008

My first field objective was to figure out the flow patterns of water through the rice field and to determine exactly how much rice-field water reaches the arsenic contaminated aquifer every year. To ascertain flow patterns, I installed water-pressure sensors at different depths in the field. Water flows from high pressure and elevation to low pressure and elevation. Knowing the relative elevation of each sensor and the pressure at their locations allows me track water movement. The picture below shows one of these sensors (the long blue rod) and a group of us debating the best installation method.

I hooked all of the sensors up to a datalogger, which is essentially a simple computer, and programmed the logger to take a measurement every hour. Therefore, even when I am back in the USA, I am collecting data (too bad my adviser does not count this as working).

The one problem with using electronic equipment at this field site is the monsoon season. During the summer, the site can be inundated with up to 15 feet of water! The rice fields are transformed into endless expanses of water. The only way to keep the datalogger dry and the sensor system working, was to house the logger above the monsoon flood level. Being an MIT student, I built a 20-foot tall tower.

The wires from the sensors run under the rice field and up the middle of the tower. Inside the cage of the tower is the datalogger and the battery that powers the sensor system. To download data off of the logger I have to climb the tower.

All of this work showed that a majority of the water in the rice field flows down the bunds, which are the raised boundaries around the edge of the rice field. Water flows down the bunds because the farmers do not plow through the bunds when they plow the rest of the field. Therefore, the soil under the bunds is less consolidated than in the rest of the rice field, and cracks under the bunds are never sealed. These factors make it easier for water to flow through the bunds than elsewhere. Bund flow is interesting because it is relatively rapid, which has implications for arsenic chemistry. In addition, if the farmers were to plow through the bunds, they could potentially save a lot of water.