Estimation of Irrigation Water Requirements

Effective irrigation management is vital for maximizing agricultural productivity while conserving water resources. In this post, we will explore the key components of estimating irrigation water requirements: crop water requirements, field irrigation requirements (FIR), farm water requirements (FWR), and diversion water requirements (DWR).

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1. Crop Water Requirements (CWR)

Crop water requirements refer to the amount of water needed by a specific crop to grow optimally throughout its growth stages. Several factors influence these requirements, including:

• Crop Type: Different crops have varying water needs. For example, rice requires more water than wheat.
• Growth Stage: Water needs change as the crop progresses through stages such as germination, flowering, and maturation.
• Climate: Temperature, humidity, and solar radiation affect evapotranspiration rates.

To estimate CWR, we typically use the following formula:

$$\text{CWR} = \text{ET} \times \text{Kc}$$

Where:

• ET = Evapotranspiration (amount of water lost from soil and plants)
• Kc = Crop coefficient (a factor reflecting the specific water needs of the crop)

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2. Field Irrigation Requirements (FIR)

Field irrigation requirements (FIR) represent the total water needed to meet the crop’s water needs within a specific field. FIR accounts for factors such as soil type, rooting depth, and effective rainfall.

To calculate FIR, consider the following:

$$\text{FIR} = \text{CWR} - \text{Effective Rainfall}$$

Where effective rainfall is the portion of rainfall that contributes to soil moisture available for the crop.

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3. Farm Water Requirements (FWR)

Farm water requirements (FWR) encompass the total water needed for all crops grown on a farm, including losses due to evaporation, runoff, and deep percolation. This is particularly important for farms with multiple crop types or fields with different water needs.

FWR can be calculated by summing the FIR for all crops and adding a buffer for expected losses:

$$\text{FWR} = \sum \text{FIR}_{\text{crop}} + \text{Losses}$$

Where losses account for irrigation system inefficiencies and environmental factors.

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4. Diversion Water Requirement (DWR)

Diversion water requirement (DWR) is the amount of water that must be diverted from a source (like a river or reservoir) to meet the FWR, accounting for any transportation losses. Understanding DWR is essential for water management and ensuring a reliable supply.

To estimate DWR:

$$\text{DWR} = \frac{\text{FWR}}{\text{Irrigation Efficiency}}$$

Where irrigation efficiency reflects how much of the diverted water is effectively used for irrigation.

Sample Problem:

            Given: Peak period of rice

                        ET_o                      = 5 mm/day

                        Crop coef             = 1.17

                        Percolation rate     = 2 mm/day

                        Effective rainfall     = 15 mm for a period of 10 days

                        Application eff         = 70%

                        Conveyance eff     = 80%

 

           

Turn out	Irrigable area
1	10 has
2	15 has
3	12 has

 

Required : Total Diversion water requirement

 

            Sol’n :

                        1). CWR = (1.17)(5 mm/day) + 2 mm/day

                                      = 5.85 mm/day + 2 mm/day

                             CWR = 7.85 mm/day

 

                        2). FIR = LPWR + CWR - RF_ef

                                                =  0 + 7.85 – (15mm/10 day)

                                    = 6.35 mm/day (1/8.64)

                                    = 0.735 lps/ha

                        3).   

                                        = 1.05 lps/ha

                        4). Diversion Water Requirement

                                    

                                    DWR = 1.3125 lps/ha

 

Turn out	Area (ha)	FWR (lps)	DWR (lps)
1	10	10.5	13.12
2	15	15.75	19.69
3	12	12.6	15.75
	= 37		DWR = 48.56 lps

 

Or 1.3125 x 37 = 48.56 lps