Díaz et al. (2025) Water balance analysis in Iowa using satellite-based evapotranspiration, precipitation, and streamflow data

Identification

• Journal: Agricultural Water Management
• Year: 2025
• Date: 2025-11-20
• Authors: M. Díaz, Antonio Arenas, Keith E. Schilling, Calvin F. Wolter
• DOI: 10.1016/j.agwat.2025.109977

Research Groups

• Department of Civil, Construction and Environmental Engineering, Iowa State University, Ames, IA, USA
• Iowa Geological Survey, University of Iowa, Iowa City, IA, USA

Short Summary

This study analyzes Iowa's water balance from 2000 to 2023 using satellite-based evapotranspiration, gridded precipitation, and streamflow data, identifying areas of groundwater-dependent evapotranspiration, validating the Budyko framework for runoff estimation, and quantifying that historical land cover change from native prairie to cropland increased annual runoff by 23%.

Objective

• To analyze the spatial distribution of satellite-based remote sensing of evapotranspiration (RSET) to precipitation (P) ratios (RSET/P) to identify areas with groundwater-dependent evapotranspiration.
• To investigate the relationship between precipitation minus RSET (P - RSET) and runoff (Q) using the Budyko framework.
• To evaluate the effects of different land cover patterns, specifically row crop agriculture versus perennial vegetation, on annual runoff within a selected watershed.

Study Configuration

• Spatial Scale: State of Iowa, USA, with detailed analysis for the Ioway Creek Watershed. Data resolutions include 30 meters for evapotranspiration and land use, and 4638.3 meters for precipitation.
• Temporal Scale: 2000 to 2023 (23-year period). Data were processed at daily, monthly, and annual (water year: October 1 to September 30) timescales.

Methodology and Data

• Models used:
  • Budyko framework (Q = P - RSET)
  • OpenET project (ensemble of six energy balance models for ET estimation)
  • Weighted arithmetic method for runoff calculation (Eq. 2)
• Data sources:
  • Satellite-based evapotranspiration (RSET): OpenET project (30 m, monthly)
  • Gridded precipitation (P): GRIDMET dataset (4638.3 m, daily)
  • In-situ streamflow (Q): National Water Information System (USGS) for Ioway Creek Watershed (daily)
  • Land use: USDA’s Cropland Data Layer (30 m, annual)
  • Watershed polygons: USGS StreamStats v4.17.0
  • Irrigation wells: Iowa Department of Natural Resources (DNR) (points)
  • Surface water irrigation withdrawals: Iowa Department of Natural Resources (DNR) (points)
  • Landforms of Iowa: Iowa Department of Natural Resources (DNR) (polygon vector)

Main Results

• During dry years, annual evapotranspiration exceeded precipitation across approximately 36,260 square kilometers of cropland in Iowa.
• Approximately 20% (6,260 square kilometers) of this area is sustained by irrigation, while the remaining 80% (30,000 square kilometers) relies on natural subsidies like stored soil moisture or shallow groundwater.
• For the Ioway Creek watershed, the Budyko framework (Q = P - RSET) served as a good proxy for observed annual runoff, showing a strong agreement (R² = 0.83, linear regression slope = 1.10).
• The historical conversion from native prairie to corn and soybean cropland increased average annual runoff by approximately 57 millimeters (23%) in the Ioway Creek watershed, primarily due to lower evapotranspiration rates of annual row crops (average annual ET for prairie: 721 mm; for cropland: 650 mm).
• Forested areas consistently demonstrated the highest average annual RSET (approximately 780 mm), followed by alfalfa and grasslands (around 720 mm), and then corn and soybeans (approximately 650 mm).
• The coefficient of variation for RSET remained consistently below 20% across the state, while for precipitation it reached as high as 50%, indicating higher year-to-year variability in precipitation than in crop water consumption.

Contributions

• Presents a replicable methodology for water balance analysis using publicly available satellite-based evapotranspiration, gridded precipitation, and in-situ streamflow data.
• Spatially identifies and quantifies non-irrigated cropland areas (30,000 square kilometers) in Iowa that are naturally resilient to drought due to access to shallow groundwater or stored soil moisture, providing valuable information for targeted agricultural management and investment.
• Validates the use of the simplified Budyko framework (P - RSET) as a valuable approach for estimating annual runoff in ungaged basins, particularly during non-extreme hydrological periods.
• Quantifies the significant long-term impact of historical land cover change (native prairie to row crop agriculture) on watershed hydrology, demonstrating a 23% increase in average annual runoff.
• Offers a valuable tool for long-term water planning and management in agricultural systems, adaptable to other regions with similar hydrological characteristics in the central United States.

Funding

• Iowa State University of Science and Technology
• USGS Water Science

Citation

@article{Díaz2025Water,
  author = {Díaz, M. and Arenas, Antonio and Schilling, Keith E. and Wolter, Calvin F.},
  title = {Water balance analysis in Iowa using satellite-based evapotranspiration, precipitation, and streamflow data},
  journal = {Agricultural Water Management},
  year = {2025},
  doi = {10.1016/j.agwat.2025.109977},
  url = {https://doi.org/10.1016/j.agwat.2025.109977}
}