Two challenges for U.S. irrigation due to climate change: increasing irrigated area in wet states and increasing irrigation rates in dry states

Abstract

Agricultural irrigation practices will likely be affected by climate change. In this paper, we use a statistical model relating observed water use by U.S. producers to the moisture deficit, and then use this statistical model to project climate changes impact on both the fraction of agricultural land irrigated and the irrigation rate (m³ ha⁻¹). Data on water withdrawals for US states (1985-2005) show that both quantities are highly positively correlated with moisture deficit (precipitation--PET). If current trends hold, climate change would increase agricultural demand for irrigation in 2090 by 4.5-21.9 million ha (B1 scenario demand: 4.5-8.7 million ha, A2 scenario demand: 9.1-21.9 million ha). Much of this new irrigated area would occur in states that currently have a wet climate and a small fraction of their agricultural land currently irrigated, posing a challenge to policymakers in states with less experience with strict regulation of agriculture water use. Moreover, most of this expansion will occur in states where current agricultural production has relatively low market value per hectare, which may make installation of irrigation uneconomical without significant changes in crops or practices by producers. Without significant increases in irrigation efficiency, climate change would also increase the average irrigation rate from 7,963 to 8,400-10,415 m³ ha⁻¹ (B1 rate: 8,400-9,145 m³ ha⁻¹, A2 rate: 9,380-10,415 m³ ha⁻¹). The irrigation rate will increase the most in states that already have dry climates and large irrigation rates, posing a challenge for water supply systems in these states. Accounting for both the increase in irrigated area and irrigation rate, total withdrawals might increase by 47.7-283.4 billion m³ (B1 withdrawal: 47.7-106.0 billion m³, A2 withdrawal: 117.4-283.4 billion m³). Increases in irrigation water-use efficiency, particularly by reducing the prevalence of surface irrigation, could eliminate the increase in total irrigation withdrawals in many states.

Figure 1. Scatterplot of estimated PET for California in 2005, using the Hamon and Blaney-Criddle metrics.

Each dot represents one year in a particular combination of GCM and greenhouse gas emissions Scenario. Note that the strong linear correlation between the two means that when either of these two metrics are statistically related to irrigation use, the quantitative predictions of the effect of climate change are quite similar. Other states also show a linear correlation, with R² ranging from 0.75 to 0.93.

Figure 2. Scatterplot of change in PET for California between 2005 and 2090, using the Hamon and Blaney-Criddle metrics.

Each dot represents one year in a particular combination of GCM and greenhouse gas emissions scenario. Note that the strong linear correlation between the two means that when either of these two metrics are statistically related to irrigation use, the quantitative predictions of the effect of climate change are quite similar. Other states also show a linear correlation, with R² ranging from 0.74 to 0.93.

Figure 3. The effect of climate change on the Hamon estimate of PET (top) and the Blaney-Criddle estimate of PET (bottom).

Both panels are colored with 5 equal interval categories that linearly span the range of the pixel values, with areas of less increase in PET being yellow and areas of greater increase in PET being red.

Figure 4. The fraction of agricultural land irrigated in U.S. states in 2005 as a function of moisture deficit.

Note the logarithmic scale on both axes. The size of circle is proportional to the irrigation rate (m³ha⁻¹); states with high moisture deficit have higher irrigation rates. The fitted regression line is shown for the middle 90% of the data range.

Figure 5. The effect of climate change on irrigation water use by United States agriculture.

A.) Historical and future trends in U.S. mean moisture deficit. Displayed is the irrigated area weighted average, which is most directly relevant to irrigation rate. For each climate change scenario, the projected moisture deficit at the climate of the ensemble median is shown. The grey area shows the range of fraction irrigated for climate of the 20^th and 80^th quintiles of the ensemble. B.) Historical and future trends in the fraction of agriculture irrigated. C.) Historical and future trends in the irrigation rate. The blue area shows the effect of climate change on irrigation rates if the current mix of irrigation equipment persists over time; the green area shows the effect of climate change if the observed (1985–2005) trend away from relatively inefficient surface irrigation continues over time. D.) Historical and future trends in total irrigation withdrawals. Note the confidence intervals of the blue (current mix of irrigation equipment) and green (decreased use of surface irrigation) areas overlap after 2030.

Figure 6. The effect of climate change on irrigation by states.

A.) Projected increase in irrigated area by 2090 under the A1B scenario, ensemble median. Most states have an increase in irrigated area under all emission scenarios in more than 80% of the GCMs in the ensemble; those that have a decrease in some cases are marked high uncertainty. B.) Projected increase in irrigation rate by 2090 under the A1B scenario, ensemble median. Most states have an increase in irrigation rate under all emission scenarios in more than 80% of the GCMs in the ensemble; those that have a decrease in some cases are marked high uncertainty.

Figure 7. Currently wet states will have significant increases in irrigated area.

The relationship between the proportion of agricultural land irrigation in 2005 and the predicted proportion of water withdrawals in 2090 (median A1B scenario of the GCM ensemble) that will come from fields not currently irrigated. A few states with significant agricultural area are labeled, using standard two-digit abbreviation for US states (see Figure 3). Three states are excluded from this graph, because climate change will have a net decrease on irrigated area there (Massachusetts, Connecticut, and Rhode Island).

Figure 8. Market value and increase in proportion irrigated.

The relationship between the average per hectare market value of cropland in 2005 and the predicted change in proportion of agricultural area irrigated (median A1B scenario of the GCM ensemble). A few states are labeled, using standard two-digit abbreviation for US states (see Figure 3).

Figure 9. Withdrawal ratios by state.

The ratio of irrigation withdrawals in 2090 to irrigation withdrawals in 2005 if the mix of irrigation technologies stays the same as today (open circles) or if the current trend away from surface irrigation continues into the future (grey squares). A ratio of more than one indicates withdrawals will increase, while a value of less than one indicates withdrawals will decrease. Data points are labeled using the standard two-digit abbreviation for US states (see Figure 3), staggered so that labels do not overlap.