Watershed, Agricultural Hydrology

John J. Ward, RG, Groundwater Consultant

Watershed hydrology is the science of the hydrologic cycle. Watershed assessments, including use of watershed models, are needed to predict the effects of changing land and water use, forest cover, and climate change on the water yield, flood frequency, and hydrologic balance within the watershed.

Numerous and important parameters that must be measured or estimated to properly calibrate a watershed model dominate the modeling process. Choice of model codes is application-specific, and may range from simple analytical solutions to computerized, highly mathematically sophisticated groundwater-watershed models. Modeling experience is critical.

Agricultural mapping and consumptive use determinations have made use of remote sensing from Landsat imagery. The normalized difference vegetation index (NDVI) technique, based on the infrared and near-infrared reflectances obtained from Landsat imagery, will give inaccurate values if not tempered with both independent information and the knowledge that certain soil moisture and crop conditions can reduce the accuracy of results.

Some representative agricultural and forested watershed projects are listed below:

Watershed Yields, Arizona and New Mexico:  Managed watershed modeling of 10 Arizona and New Mexico forest watersheds for seasonal yields and stream base flow due to decadal watershed drought conditions. Investigation included determination of historical meteorological variability, assessment of the role of snowpack in maintaining riparian areas, modeling watershed dynamics using Soil and Water Assessment Tool (SWAT) watershed model.

Watershed in high mountain terrain exhibits dominance of elevation, aspect, and snow pack depletion on both function and yield.
Subwatershed definitions in southern Rocky Mountain drainage. First step in delineation of hydrologic response units for watershed.
  • Watershed Flooding, Florida:  Constructed, installed and calibrated permanent stream gages to measure baseline watershed conditions prior to mining. Developed watershed flow balances by installing and maintaining stream and precipitation gages in heavily mined watersheds.
  • Agricultural Water Use Optimization, Arizona:  Repaired degraded well/pump system from effects of calcium deposition through pump and acid control, and optimized water and fertilizer delivery to 2,000+ acre drip irrigation system.
  • Agricultural Watershed Yields, New Mexico:  Constructed watershed models of six forested watersheds, New Mexico, using Precipitation-Runoff Modeling System (PRMS) code, to determine variations in seasonal agricultural water yields.
  • Agricultural Lands Remote Sensing, New Mexico:  Using Landsat imagery over a 15-year period, determined seasonal irrigated agricultural acreages for eight agricultural districts, using normalized difference vegetation indices (NDVI) and air photo analysis. Developed methodology to correct acreages for fallow periods and deficit irrigation, as well as for tail water and off-plot growth.
  • Irrigation Use Remote Sensing, New Mexico:  Conducted review analysis of NDVI calculations of irrigated acreages from different irrigation methods, southern New Mexico.
  • Spring and Streamflow Recession, Arizona:  Analyzed streamflow recession hydrographs to determine floodflow, snowmelt and baseflow components from streams, Northern Arizona. Data used to quantify water balances and calibrate numerical groundwater – surface water model.
  • National Park Service Watershed Monitoring, Arizona:  Conducted hydrologic budget analysis of riparain 2,000 acre watershed, Central Arizona. Projected included installation of weather station and evaporation pan for energy balance determinations, monitoring influent and effluent streamflow, spring discharges, and wetland water levels. Calculated hydrologic balance using energy and water budgets.
Meteorological station installed for watershed analysis, Tuzigoot Nat Mon, Ariz.