Methods 5.0
Irrigation helps maximize crop production, especially in regions where rainfall alone is insufficient to meet crop requirements. Agricultural irrigation is also the largest consumer of water in the US (Sowby et al. 2024) . Water resources must therefore be monitored and managed sustainably (Sears et al. 2018). In the Fieldprint Platform, Irrigation Water Use (\(IWU\)) is an indicator of how efficiently water was used for crop irrigation.
The \(IWU\) metric is defined as the amount of water applied for a marginal increase in crop yield attributed to irrigation. As an efficiency metric, lower \(IWU\) values are desirable and result from either using less irrigation water or from producing significantly greater yields due to added irrigation.
This metric is only calculated only on fields that received irrigation.
The value for water applied is determined from the total amount used, from all sources, over the time of the crop interval. 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.
| 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 | Depth of annual irrigation water applied from all sources | acre-inch; \({ac\mbox{-}in}\) |
\(W\) |
| Conversion factor for acre-inch to cubic meters3 | 102.79031 | \(\frac{m^{3}}{ac\mbox{-}in}\) |
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.
3 If interested in total water volume, need to remember the area conversion from acres to hectares.
\[IWU = \frac{W}{Y_i - Y_0}\]
# function for demonstrating Irrigation Water Use calculation
# w = total water applied from all sources,
# yi = yield with irrigation (at standard moisture)
# y0 = yield without irrigation (default = 0)
water_use <- function(w, yi, y0 = 0) {
# Calculate irrigation water use (iwu)
iwu = w / (yi - y0)
return(iwu)
}The farmer applied 12 inches of water per acre throughout the growing season and harvested 60 bu/acre of soybean. Nearby fields were not irrigated and only yielded of 40 bu/acre. Calculated the \(IWU\) value for this irrigated field.
# Calculation in USCS Units
iwu = water_use(
w = 12, # acre-inches
yi = 60, # bu/ac at standard moisture
y0 = 45 # bu/ac at standard moisture
)
# Convert to SI units
iwu_metric <- water_use(
w = 12 * acreinch_m3,
yi = 60 * 60 * lb_kg,
y0 = 45 * 60 * lb_kg)[1] "0.8 acre-inch for each additional bushel/acre"[1] "3.02 m^3 for each additional kg/ha"A grower used 18 acre-inch of surface water and 6 acre-inch of groundwater to irrigated their rice field throughout the crop interval. They harvested 70 cwt/ac of rice. Calculated the \(IWU\) value for this irrigated field.
# Calculation
iwu <- water_use(
w = 18 + 6,
yi = 70 * 100) # convert cwt to lbs
iwu_metric <- water_use(
w = (18 + 6) * acreinch_ac_m3_ha,
yi = 70 * 100 * lb_kg / ac_ha)[1] "0.0034 acre-inch for each additional lb/acre"[1] "0.777 m^3 for each additional kg/ha"