Abstract
The Colorado River Basin (CRB) includes seven states and provides municipal and industrial water to millions of people across all major southwestern cities both inside and outside the basin. Agriculture is the largest part of the CRB economy and crop production depends on irrigation, which accounts for about 74% of the total water demand cross the region. A better understanding of irrigation water demands is critically needed as temperatures continue to rise and drought intensifies, potentially leading to water shortages across the region. Yet, past research on irrigation dynamics has generally utilized relatively low spatiotemporal resolution datasets and has often overlooked the relationship between climate and management decisions such as land fallowing, i.e., the practice of leaving cultivated land idle for a growing season. Here, we produced annual estimates of fallow and active cropland extent at high spatial resolution (30 m) from 2001 to 2017 by applying the fallow-land algorithm based on neighborhood and temporal anomalies (FANTA). We specifically focused on diverse CRB agricultural regions: the lower Colorado River planning (LCRP) area and the Pinal and Phoenix active management areas (PPAMA). Utilizing ground observations collected in 2014 and 2017, we found an overall classification accuracy of 88.9% and 87.2% for LCRP and PPAMA, respectively. We then quantified how factors such as climate, district water rights, and market value influenced: (1) annual fallow and active cropland extent and (2) annual cropland productivity, approximated by integrated growing season NDVI (iNDVI). We found that for the LCRP, a region of winter cropping and senior (i.e., preferential) water rights, active cropland productivity was positively correlated with cool-season average vapor pressure deficit (R = 0.72; p < 0.01). By contrast, for the PPAMA, a region of summer cropping and junior water rights, annual fallow and active cropland extent was positively correlated with cool-season aridity (precipitation/potential evapotranspiration) (R = 0.46; p < 0.05), and active cropland productivity was positively correlated with warm-season aridity (precipitation/potential evapotranspiration) (R = 0.42; p < 0.01). We also found that PPAMA cropland productivity was more sensitive to aridity when crop prices were low, potentially due to the influence of market value on management decisions. Our analysis highlights how biophysical (e.g., temperature and precipitation) and socioeconomic (e.g., water rights and crop market value) factors interact to explain seasonal patterns of cropland extent, water use and productivity. These findings indicate that increasing aridity across the region may result in reduced cropland productivity and increased land fallowing for some regions, particularly those with junior water rights.
Highlights
Irrigated agriculture accounts for approximately 70% of global annual freshwater consumption and 40% of global annual crop production, enabling stability in food and feed production across water-limited regions [1,2,3,4]
Based on ground observations collected in 2014 and 2017, we found overall classification accuracies of 88.9% and 87.2% for the lower Colorado River planning (LCRP) and Pinal and Phoenix active management areas (PPAMA), respectively (Figure 2; Tables S2 and S3)
Our approach is self-calibrating based on simple inputs and spatiotemporal statistical analysis of greenness, and it produces annual spatially explicit fallow and active crop extent maps that provide consistent tracking of irrigation, which can supplement or support current cropland classifications for both scientific and managerial applications
Summary
Irrigated agriculture accounts for approximately 70% of global annual freshwater consumption and 40% of global annual crop production, enabling stability in food and feed production across water-limited regions [1,2,3,4]. For many regions, current rates of irrigation are unsustainable and putting significant pressure on regional freshwater supplies [5], especially as food demand increases [6] and drought intensifies [7,8]. With current high demand for water within the Colorado River Basin (CRB) in the United States, published estimates of water supply indicate periods of shortages as temperatures warm without increased precipitation [3,9,10,11], which will likely lead to failure in meeting water allocation demand [3]. Crop prices, water policies, crop rotation and the dynamics of fallowed agriculture all drive irrigated cropland interannual variability [12,13,14,15]. Climate change may affect crop yields and regional agriculture patterns both directly and via management decisions to change crop type or fallow fields [16,17], and these crop management decisions impact communities’ economies, food security and water availability [17,18]
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