THIS presentation is intended to help rice farmers to cope with water scarcity at the field level. The way to deal with reduced (irrigation or rain) water inflows to rice fields is to reduce the nonproductive outflows by seepage, percolation, or evaporation, while maintaining transpiration flows (as these contribute to crop growth). This can be done at land preparation, at crop establishment, and during the actual crop growth period.

Land preparation
Land preparation lays the foundation for the whole cropping season and it is important in any situation to “get the basics right.” Especially important for good water management are field channels/drains, land leveling, and tillage operations (puddling, and meer/bund preparation and maintenance).

Field Channels
Many irrigation systems in Guyana have no field channels (or “tertiary” irrigation or drainage channels) and water flows from one field into the other through breaches in the bunds. This is called “plot-to- plot” irrigation. The amount of water flowing in and out of a rice field cannot be controlled and field-specific water management is not possible. This means that farmers may not be able to drain their fields proerly before harvest because water keeps flowing in from other fields. Also, they may not be able to have water flowing in if upstream farmers retain water in their fields or let their fields dry out to prepare for harvest. Moreover, a number of technologies to cope with water scarcity require good water control for individual fields. Finally, the water that continuously flows through rice fields may remove valuable (fertilizer)

Tillage: Reducing soil permeability
Seepage and percolation flows from rice fields are governed by the permeability (hydraulic conductivity) of their soils: their capacity to conduct water downward and sideward. A rice field can be compared to a bathtub: the material of a bathtub is impregnable and it holds water well—however, if you have only one hole (by removing the plug), the water runs out immediately. Rice fields just need a few rat holes or leaky spots and they will rapidly lose water by seepage and percolation. Large amounts of water can be lost during soaking prior to puddling when large and deep cracks are present that favor rapid “by-pass flow” to below the root zone. Thorough puddling results in a good compacted plow sole that reduces permeability and percolation rates throughout the crop growing period. The efficacy of puddling in reducing percolation depends greatly on soil properties (which was discussed in my previous article). Puddling may not be effective in coarse soils, which do not have enough fine clay particles to migrate downward and fill up the cracks and pores in the plow sole.

On the other hand, puddling is very efficient in clay soils that form cracks during the fallow period that penetrate the plow pan. Although puddling reduces percolation rates of the soil, the action of puddling itself consumes water, and there is a trade-off between the amount of water used for puddling and the amount of water “saved” during the crop growth period by reduced percolation rates. Puddling may not be necessary in heavy clay soils with low vertical permeability or limited internal drainage. In such soils, direct dry seeding on land that is not puddled but tilled in a dry state is very well possible with minimal percolation losses. Soil compaction using heavy machinery has been shown to decrease soil permeability in sandy soil with loamy subsoils with at least 5% clay. Although most farmers cannot afford to compact their soils, this technology may be feasible on a large scale with government support.

Bund/Meer preparation and maintenance
Good bunds are a prerequisite to limit seepage and underbund flows. To limit seepage losses, bunds should be well compacted and any cracks or rat holes should be plastered with mud at the beginning of the crop season. Make bunds high enough (at least 20 cm) to avoid over-bund flow during heavy rainfall. Small levees of 5–10-cm height in the bunds can be used to keep ponded water depth at that height. If more water needs to be stored, it is relatively simple to close these levees.

Crop establishment
Minimizing the turnaround time between land soaking for wet land preparation and transplanting reduces the period when no crop is present and when outflows of water from the field do not contribute to production. Especially in large-scale irrigation systems with plot-to-plot irrigation, water losses during the turnaround time can be very high. For instance, let’s say MMA/ADA the largest surface irrigation scheme in Guyana, it can take many days from the first day of delivery for land preparation until the whole area is transplanted. From the total amount of water delivered 50% can be lost due to seepage, percolation and evaporation.

Saturated soil culture
In saturated soil culture (SSC), the soil is kept as close to saturation as possible, thereby reducing the hydraulic head of the ponded water, which decreases the seepage and percolation flows. SSC in practice means that a shallow irrigation is given to obtain about 1 cm of ponded water depth a day or so after the disappearance of ponded water. Although conceptually sound, SSC will be difficult to implement practically since it requires frequent (daily or once every two days) applications of small amounts of irrigation water to just keep a standing water depth of 1 cm on flat land, or to keep furrows filled just to the top in raised beds. But can be an alternative if the situation become worsen.

Alternate wetting and drying
In alternate wetting and drying (AWD), irrigation water is applied to obtain flooded conditions after a certain number of days have passed after the disappearance of ponded water. The number of days of non-flooded soil in AWD before irrigation is applied can vary from 1 day to more than 10 days. AWD increased water productivity (WPIR) with respect to total water input because the reductions in water inputs were larger than the reductions in yield. More water can be saved and water productivity further increased by prolonging the periods of dry soil and imposing a slight drought stress on the plants, but this usually comes at the expense of a slight yield loss. AWD is a mature technology that has been widely adopted in China, India and Philippines. Very little research has been done in Guyana to quantify the impact of AWD on the different water outflows of rice fields: evaporation, seepage, and percolation. I recommend that research work be initiated in this area so farmers can benefit from the results.
Bissasar Chintamanie
Research Scientist