Virtual Water Storage and Flows within the Continental United States
Advisor: Associate Professor Megan Konar
Meeting ID: 862 7775 6173
Agriculture is the largest user of water in the United States. Yet we do not understand many
details about the virtual water content embedded across various parts of the agricultural supply
chain. Like physical hydrology, a more robust understanding of both flows and stocks of virtual
water resources must be considered to obtain an accurate representation of the system. Regarding
the state of research available prior to this dissertation, virtual water storage had never been
studied in detail, and virtual water transfers had never been constructed and analyzed at the
United States county scale. Additionally, to quantify virtual water transfers more accurately, a
better understanding of spatially resolved sources of irrigation water use by crop was needed.
Here we address these issues by: 1). Quantifying Virtual Water Storage Capacities (VWSC)
across the Continental United States (CONUS) for the years 2002, 2007 and 2012; 2). Modeling
Irrigation Water Use (IWU) of various crops across the CONUS from 2008-2020; and 3).
Combining IWU data with downscaled food trade data to calculate Virtual Water Content
(VWC) and Virtual Water Transfers (VWT) of three crop types across three water sources for
the years 2012 and 2017 within the CONUS. To address these questions, we employ a variety of
data-intensive approaches in which a variety of government databases are synthesized and
integrated into pre-existing hydrological modeling structures to arrive at our results. We find
that 728 km3 of water could be stored as grain in the United States, with roughly 86% coming
from precipitation. National VWSC capacities were 777 km3 in 2002, 681 km3 in 2007, and 728
km3 in 2012. This represents a 6% decline in VWSC over the full 10-year period, mostly
attributable to increased water productivity. VWSC represents 62% of U.S. dam storage and
accounts for 75–97% of precipitation receipts to agricultural areas, depending on the year.
Regarding IWU estimates between 2008-2020, Surface Water Withdrawals (SWW) decreased by
20%, while both Groundwater Withdrawals (GWW) and Groundwater Depletion (GWD)
increased by 3%. On average, animal feed (alfalfa/hay) uses the most irrigation water across all
water sources: 33 km3/year from SWW, 13 km3/year from GWW, and 10 km3/year from GWD.
Produce used less SWW (43%), but more GWW (57%) and GWD (27%) over the study timeperiod.
The most significant changes for each water source by crop are: rice (SWW decreased by
71%), sugar beets (GWW increased by 232%), and rapeseed (GWD increased by 405%).
Including both surface and groundwater contributions to irrigation water, we estimate state
averages of 151 m3/ton of blue VWC in cereal grains, 478 m3/ton in produce crops, and 351
m3/ton in animal feed in 2017 (211 m3/ton, 944 m3/ton, and 363 m3/ton in 2012, respectively).
Produce crops changed the most over the period in terms of state averaged VWC (49%
decrease), followed by cereal grains (28% decrease) and animal feed (3% decrease), resulting in
average VWC decreases in all three crop categories over the five years. Considering VWT, we
see more variance in the changes between 2012 and 2017. Total VWT from blue water sources
in cereal grains was 199 km3, produce was 130 km3, and animal feed was 93 km3 in 2017. The
same values in 2012 were 56 km3 for cereal grains, 175 km3 for produce, and 108 km3 for animal
feed. This means cereal grain total VWT changed the most over the period (255% increase). The
other two crop categories’ VWT instead increased, with produce having the larger decrease (26%
decrease) followed by animal feed (14% decrease). This work collectively enhances our
understanding of the food-water nexus and enables future work in virtual water to integrate both
storage and flow values into trade models. Our results also highlight the need for proactive water
management and planning and underscore the long-term risks that reliance on unsustainable
irrigation poses to the national food supply and greater agricultural supply chain.