Irrigation design philosophy – be with nature, not against nature
The overall objective is to control water levels during the growth period of the rice. There are many factors affecting irrigation design and construction such as agronomy, natural conditions of the site.
As much as possible we will time the cropping seasons to coincide with the rainy season. If any top up water is required, this will be done through pumping/gravity irrigation. This way we can take advantage of the rainy season to save pumping costs.
If the soil is high in clay it is suitable for earth canals. We will only need to reinforce the canals in certain places which are not able to hold water. This is how we control irrigation costs in the initial period. As we expand the operation there might be a need for more permanent structures in place.
Gravityirrigation will be used where possible. We will use existing streams as drainage and construct separate water supply canals and internal field canals.
Gravitysurface irrigation flows
The diagrams illustrates a simple surface water irrigation system using gravity. Given the flow direction of the river the land would be naturally sloping south-west.
Pumping station A will supply the field through a system of main canals surrounding the land. Smaller canals and gates will let water into the partitioned fields.
Pumping station B will act as both water supply and drainage (e.g. during rainy season). Tail water will be at point B given the topography of the
Topographical, hydrological survey
Topographicalconditions determine scheme layout, specially when gravity irrigation is considered (building a reservoir and dam).
Topography outlines the surface conditions and is therefore essential for the layout and design. Data accuracy should increase with each project development step (e.g., site selection, pre-feasibility, feasibility, etc.).
Usually, site topographic data is presented in a topographic map with a specified scale (e.g., 1:500), and a digital elevation model
Hydrologicalconditions determine the flow available for irrigation and the dimensions of intake structures, spillways and waterways.
It is determined through a hydrological study that considers flow measurements, catchment area, precipitation, and flow distribution during the year.
A satisfactory statistical analysis of the hydrology of potential sites requires a period of at least 15 years of flow monitoring or precipitation monitoring. A flow duration curve (FDC) represents available flow and its distribution
Crop water requirement calculations
Inflows = Rain + Irrigation + Capillary rise + Overbundinflow + Seepage from higher field
Outflows = Transpiration by the crop + Evaporation + Percolation + Overbundoutflow + Seepage into lower field
Rice water requirement
Lowland rice requires a lot of water. Rice is typically grown in bundedfields that are continuously flooded up to 7−10 days before harvest.
Continuous flooding helps ensure sufficient water and control weeds. In Vietnam's experience, each ha of rice requires more than 10,000m3of water a year for 2 – 3 cropping cycles.
For lowland rice, water is needed to prepare the land and to match the outflows seepage, percolation, and evapotranspiration during crop growth (see figure). The amount of water used for wet land preparation can be as low as 100–150 mm (depth of water per surface area) when the time lag between soaking and transplanting is only a few days or when the crop is directly wet seeded.
Seepageis the lateral subsurface flow of water, and percolationis the flow of water down below the root zone. Typical combined values for seepage and percolation vary from 1–5 mm a day in heavy clay soils to 25–30 mm a day in sandy and sandy loam soils.
Evaporationis water lost into the air as vapor from the ponded water layer or from the surface of the soil, and transpiration is water released into the air as vapor through the plants. Typical combined evapotranspiration rates of rice fields are 4–5 mm a day in the wet season and 6–7 mm a day in the dry season.
Over-bund flow or surface runoff is the spillover when water depths rise above the bunds of the fields.
Seepage, percolation, evaporation, and over-bund flow are all nonproductive flows of water and are considered losses at the field level.
The water requirements of crops will be calculated using the FAO's CROPWAT. Water requirements and irrigation requirements were estimated using soil, climate and rice crop data. FAO CROPWAT allows the development of irrigation schedules for different water management conditions and irrigation practices under both rain-fed and irrigated conditions. Crop water requirements are calculated based on the Crop Evapotranspiration.
The determinant of irrigation design
Example of analysis
Topographically the area has relatively large slope, starting from the main canal sloping down on both sides and from the head of the main canal sloping down to the foot at a slope of 0.5% to 5%. As such this represents a challenge in leveling the plots. Also the top soil layer is not thick, only from 0.5-1m and is mostly sandy soil with very low clay content (<20%) which is very difficult for prevention of absorption, as well as keeping water in the plots.
We need to set up plots at different altitudes (like steps on a staircase), the size of each plot depends on the slope (as said above the top soil is less than 1m).
Also we need ensure minimum width of each plot is 20m for agriculture machinery to operate. We can only level the plots with a maximum cross-section thickness of 50cm. Therefore we can only set up plots that meet the above requirements in an area with maximum slope of 0.5m/(20+5)m = 2%
Therefore plots with slope > 2% will be excluded from the irrigated area.