New study: Maps of western water supply overestimate reality

In the western United States, water is more than a resource—it’s a legal battleground. Surface water users rely on a system of water rights known as the Prior Appropriations Doctrine, which dictates that “first in time, first in right” determines who gets water during shortages. But how well do existing maps and models reflect the actual connection between land and water rights?

A new study by Megan Wilson Schantz at Virginia Tech suggests that current mapping methods may significantly underestimate the extent of appropriated water rights—potentially overstating how much unclaimed water exists. Her research, linking water rights to land parcels, could have major implications for water management and agricultural planning in the face of climate change.

What’s New About This Study?

Mapping surface water rights to irrigated acreage and land parcels is nothing new for right holders proving their claims because of litigation or for other reasons.  Regarding planning, however, most research on water availability in the West focuses on climate models—tracking precipitation, temperature changes, and drought risks.  Schantz argues that these studies overlook the legal framework governing water distribution. Her work takes a different approach:

• She pairs water rights to specific land parcels rather than looking at them as separate systems.
• She analyzes crops at both state and hydrologic levels to determine which fields might lose water first during droughts.
• She uses a global hydrologic model to compare theoretical water allocation with how the prior appropriations system realistically works.

The key finding? When water rights were paired with land parcels using spatial overlays, the estimated water rights coverage increased from 29% to 59% across ten western states. This suggests that past mapping efforts may have underrepresented the true extent of legally claimed water, potentially leading to flawed assumptions about available supplies.

Which Crops Hold the Strongest Water Claims?

By examining the median priority dates for different crops, Schantz found that:

• Oats and other hay have the most senior water rights, meaning their growers are first in line for water even in a drought.
• Corn growers often hold junior water rights, making them more vulnerable to shortages.
• Other crops showed less clear patterns in water rights seniority.

This hierarchy matters because priority determines survival. When drought hits, senior rights holders get their full water allotment first—potentially leaving newer farms high and dry.

Why This Research Matters.

The study provides a much-needed bridge between legal water rights and actual land use, giving policymakers, farmers, and water managers a more accurate tool for predicting shortages and conflicts. It also raises the question: Are current maps and water models misleading policymakers into thinking more water is available than legally exists?

With climate change intensifying water scarcity in the West, understanding the real extent of water rights—and how they align with land use—could shape future water policy, agricultural practices, and even urban planning. Schantz’s methodology offers a framework for integrating water rights into global hydrologic models, bringing much-needed legal reality into scientific forecasting.

In short, Schantz’s premise is that if we are to manage water wisely, we need to start by mapping it accurately.