Case Study

Investigation of Water Management in Rice Paddies

 Rice cultivation in wet paddies seems to involve simply filling paddies with water and allowing rice to grow. However, in reality, a great deal of effort is focused on water management, with changes in water depth or drainage from planting to reaping being particularly important. The methods of water management in rice paddies vary with differences in factors such as regional precipitation, irrigation, and water infiltration rate. Therefore, we examined specific water management methods identified during interviews of farmers from 7 locations in Japan, in order to identify regional differences.

1. Basis of Water Management
The first requirement for the cultivation of rice is reliable water supply. In addition, rice must be protected from heat and cold, the growth of weeds controlled, and rice tillering facilitated. However, rice rot may occur more often in flooded paddies due to oxygen deprivation in the submerged parts of the plant. Other disadvantages include the potential softening of soil that may allow rice to fall over more easily, making it difficult to harvest the crop. With respect to vegetative growth, over-tillering is a problem. Therefore, if wet rice cultivation is to proceed smoothly, it is important to balance the advantages and disadvantages of using standing water (i.e., water supply and oxygen supply must be balanced). Through several trial-and-error efforts in the past, conventional rice cultivation practice includes detailed management strategies, such as controlled flooding and draining, according to the needs of a particular region. Water management technology is a very prominent form of wet rice cultivation technology.
2. Standard Water Management Methods
A typical water management method is shown in the figure below. During planting, the paddy is flooded to a depth of about 35 mm, and soil is puddled. Because soil dries up in winter, it is necessary to irrigate a paddy with a volume 3–5 times the capacity of the paddy. After planting, when the seedlings have taken root, the water level is lowered to about 25 mm to facilitate tilling. Then, weed control is performed. Previously, this process was repeated three times during each growing cycle, but carrying out all controls at once is now the mainstream. At this time, paddies are deeply flooded as it is necessary to allow paddies to lie undisturbed for a time.
Incidentally, because of continuous water infiltration and evaporation, the water level in rice paddies drops down to 10–30 mm each day even in the absence of active water management. Although the rate of daily water loss varies by farm, a summary of the reports of local farmers indicates that 15 mm of water loss a day may be standard. Under these circumstances, when paddies are flooded to a depth of 60 mm, all standing water disappears in 4 days. It is ideal to apply herbicides for about a week, but because allowing paddies to dry out completely has negative effects, water is added as soon as the standing water is depleted and water management shifts to a shallow flooding regime.
Approximately 40 days after planting, midseason drainage is carried out, wherein all the water is drained and the paddies are allowed to dry. The purpose of midseason drainage is to increase the oxygen supply to the roots, to prevent over-tillering, to decrease the rate at which rice plants fall over, and to make harvesting processes easier by allowing the soil to harden. However, during midseason drainage the paddies are prevented from drying out completely, by adding just enough water to wet the soil when small cracks appear. If paddies dry out extensively (when bigger cracks appear), water infiltration rates drop, making the water level much more difficult later. Conversely, during prolonged monsoons the soil may not dry at all, as this growth stage takes place during the rainy season in Japan. This is then managed by digging the paddies in order to improve drainage. Midseason drainage is typically carried out for 10 days to 2 weeks.
Following midseason drainage, shallow flooding management or intermittent irrigation is carried out until the heading stage. Intermittent irrigation involves periodic irrigation of paddies whenever standing water is depleted. Rice plants are much more susceptible to drying in this season, but at the same their roots must have access to oxygen. Knowledge of proper irrigation and draining practices is a significant part of rice cultivation, and the word “kakehiki” is said to be derived from it. In some cases, a high water level is maintained during the heading stage, as consistent access to water is especially necessary during this period. These water management practices are implemented until harvest.

Typical water management in rice cultivation