Irrigation Water Use

Methods 5.0

Acre-inches of water applied for each incremental increase in crop yield attributed to irrigation.

Introduction

Irrigation Water Use (\(IWU\)) is an efficiency metric representing the amount of water applied (unit = acre-inch) for each incremental increase in crop yield attributed to irrigation. Lower \(IWU\) values result from either using less irrigation water or from greater yield returns due to added irrigation.

This metric is calculated only for users who irrigate. No water efficiency metric applies to rainfed crop production.

Methods

The water applied is determined from the total amount from all sources. Estimates for non-irrigated yield can be as low as zero, or a total failure of the crop. For example with rice, users may enter water applied from more than one irrigation source. If so, sum the total water applied from all sources to get \(W\). For rice, non-irrigated yield is assumed to be zero and does not need to be entered by the user.

While still a work in progress, Field to Market wants to support users by developing better methods to estimate non-irrigated yields.

Inputs

Input	Value	Units	Symbol
Irrigated yield1	User entry, converted to standard moisture		\(Y_i\)
Non-irrigated yield2	User estimate of yield had there been no irrigation; converted to standard moisture		\(Y_0\)
Total irrigation amount	Annual irrigation water applied from all sources	acre-inch	\(W\)
Conversion factors for acre-inch to cubic feet	3630	acre-inch/ft3
Conversion factors for cubic feet to cubic meters	0.0283168	ft3/m3

¹ Any adjustments made to crop yield calculations to reconcile moisture content (rice, alfalfa) or sugar content (sugar beets) should also be made to the “non-irrigated yield”. Yields must be in the same units prior to calculation of the Irrigation Water Use metric.

² Can be zero.

Formula

\[IWU = \frac{W}{Y_i - Y_0}\]

Supporting R function

# function for demonstrating Irrigation Water Use calculation
# w  = total water applied from all sources,
# yi = yield with irrigation
# y0 = yield without irrigation (default = 0)

water_use <- function(w, yi, y0 = 0) {
  # Calculate irrigation water use (iwu)
  iwu = w / (yi - y0)
  
  return(iwu)
}

Example: Soybean🌱

An irrigated soybean field with an irrigated yield of 60 bu/acre and an estimate of non-irrigated yield of 45 bu/acre and 12 acre-inches of water applied.

# Inputs
yield_adjusted_uscs_irrigated <- 60  # standardized yield with irrigation
yield_adjusted_uscs_not_irrigated <- 45  # standardized yield without irrigation

gross_water_pumped <- 12    # water use (acre-inch)

# Calculation
iwu = water_use(w = gross_water_pumped,
                yi = yield_adjusted_uscs_irrigated,
                y0 = yield_adjusted_uscs_not_irrigated)

iwu_metric <- water_use(w = gross_water_pumped * acreinch_m3,
                        yi = yield_adjusted_uscs_irrigated * 60 * lb_kg / ac_ha,
                        y0 = yield_adjusted_uscs_not_irrigated * 60 * lb_kg / ac_ha)

[1] "0.8 acre-inch for each additional bushel/acre"

[1] "1.22 m^3 for each additional kg/ha"

[1] "1,223 m^3 for each additional Mg/ha"

Example: Rice🌾

A rice field with a standardized irrigated yield of 70 cwt from two sources of water: 18 acre-inch were sourced from surface water 6 acre-inch were sourced from groundwater.

# Calculation
iwu <- water_use(w = 18 + 6,
                 yi = 70 * 100) # convert to lbs

iwu_metric <- water_use(w = (18 + 6) * acreinch_m3,
                        yi = 70 * 100 * lb_kg / ac_ha)

[1] "0.0034 acre-inch for each additional lb/acre"

[1] "0.314 m^3 for each additional kg/ha"