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USGS and Mohave County update model: Kingman subbasin shows large agricultural drawdown, hundred‑year adequacy at risk
Summary
Mohave County and the U.S. Geological Survey presented a 2024 update to the Hualapai (Wallapai) Valley groundwater model showing a substantially larger agricultural withdrawal and a modeled deficit of roughly 50,000 acre‑feet annually in the basin, with the Kingman subbasin most affected.
Mohave County staff and hydrologists from the U.S. Geological Survey presented updated observations and modeling of the Hualapai (sometimes referred to in the presentation as Wallapai) Valley groundwater basin on June 16, reporting that irrigated acreage—particularly in the Kingman subbasin—has increased since the county’s 2019 observations and that modeled annual deficit has grown.
Key findings were summarized by Scott (Development Services) and Jamie Macey of the USGS: aerial and drone surveys in 2024 identified approximately 16,505 acres of irrigated orchards and vines countywide in the basin (an increase concentrated in the Kingman subbasin), producing an estimated agricultural demand of about 44,252 acre‑feet. Combining recharge and withdrawals, staff reported a modeled annual basin deficit of about 50,474 acre‑feet—up from about 30,900 acre‑feet reported in the 2021 model run that used 2019 data.
Why it matters: county and city utilities draw water from the Kingman subbasin; model projections show declining groundwater levels that could reach benchmarks used by Arizona Department of Water Resources for adequacy (1,200 feet depth to water) within the model century if withdrawals continue on the current trajectory. County and USGS staff emphasized the model is a living tool that improves with additional monitoring wells and updated use data.
What the presentations covered - USGS and county staff reviewed methodology: 2019 aerial basemap used in the 2021 study was augmented by 2024 drone and aerial observations to map irrigated acres and crop types (orchards and vines were treated with crop‑type consumptive‑use rates). The county used average consumptive‑use rates (for orchards ~2.72 acre‑feet/acre; vines ~1.01 acre‑feet/acre) to estimate irrigation demand. - Results showed a particularly large increase in cultivated acres in the Kingman subbasin compared with 2019 (+~6,800 acres) while other subbasins varied; the Kingman subbasin accounts for most of the new demand and is the location of municipal supply wells. - Model outputs include projected well‑pumping decline curves and an estimated increase in average depth to groundwater if current patterns persist; the Kingman subbasin’s average depth was cited near 600 feet now and the model shows progressive decline toward deeper levels across the projection horizon.
County response and next steps County Manager Elders and development staff said they have started to install or plan monitoring wells and recharge basins to gather better data and provide limited infiltration where feasible. The county has begun drilling monitoring wells with USGS partnership; previously installed monitoring wells had been vandalized and required replacement, staff said.
USGS context Jamie Macey (Arizona Water Science Center) said the USGS developed the original model in cooperation with the county based on 2019 data and reaffirmed that the USGS does not manage water rights; its role is to provide data and modeling to inform management decisions. Macey encouraged continued updating of the model as county field observations improve.
Ending: County staff asked the board to treat the groundwater model as an ongoing planning tool and to support monitoring and recharge projects; supervisors asked staff for parcel ownership detail on irrigated acres and emphasized the need to coordinate city and county planning with water resource projections.
