Steps for rice irrigation requirement

Irrigation water requirement (IWR) of rice is the water that must be supplied by irrigation, after deducting effective rainfall, to meet crop evapotranspiration, field losses and land preparation needs.

Steps

1. Collect data: climate (temperature, humidity, wind, sunshine), rainfall, soil type (percolation), crop calendar (nursery, transplanting, growing stages, harvest).
2. Reference evapotranspiration ($ET_o$): compute by Penman, modified Penman or Blaney-Criddle method for each month or ten-day period.
3. Crop coefficient ($K_c$): select $K_c$ for each growth stage of rice (initial, development, mid, late) from FAO tables.
4. Crop evapotranspiration:

$$ET_c = K_c \times ET_o$$

5. Seepage and percolation (S&P): estimate daily loss from the standing water (e.g. 2-6 mm/day depending on soil).
6. Land preparation water (puddling): estimate water for soaking and puddling, typically 150-250 mm, spread over the preparation period; also nursery requirement.
7. Water layer (WL): water needed to establish and maintain standing water, e.g. 50-100 mm at transplanting.
8. Effective rainfall ($P_e$): estimate by dependable rainfall (75% probability), USDA SCS or fixed percentage method.
9. Net irrigation requirement:

$$\begin{aligned} NIR = ET_c + S\&P + WL + LP - P_e \end{aligned}$$

10. Gross irrigation requirement:

$$GIR = \frac{NIR}{\eta_{overall}}$$

where $\eta_{overall}=\eta_c\times\eta_a$ (conveyance and application efficiencies). 11. Convert to discharge: the peak monthly GIR gives the canal capacity (l/s/ha, or $m^3/s$ for the command area).