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Pesticide Root Zone Model (PRZM)

The EPA's Pesticide Root Zone Model (PRZM) is a one-dimensional, dynamic, compartmental model that can be used to simulate chemical movement in unsaturated soil systems within and immediately below the plant root zone. It has two major components-- hydrology (and hydraulics) and chemical transport. The hydrologic component for calculating runoff and erosion is based on the Soil Conservation Service curve number technique and the Universal Soil Loss Equation. Evapotranspiration is estimated either directly from pan evaporation data, or based on an empirical formula. Evapotranspiration is divided among evaporation from crop interception, evaporation from soil, and transpiration by the crop. Water movement is simulated by the use of generalized soil parameters, including field capacity, wilting point, and saturation water content.

The chemical transport component can simulate pesticides or organic and inorganic nitrogen species. For pesticides, the transport component can simulate pesticide application on the soil or on the plant foliage. Biodegradation can also be considered in the root zone. Dissolved, adsorbed, and vapor-phase concentrations in the soil are estimated by simultaneously considering the processes of pesticide uptake by plants, surface runoff, erosion, decay, volatilization, foliar washoff, advection, dispersion, and retardation. For nitrogen, simulation of surface applications, atmospheric deposition, and septic effluent discharge may all be simulated. The nitrogen species of nitrate, ammonia, and four forms of organic nitrogen (i.e. particulate organic nitrogen (labile and refractory) and dissolved organic nitrogen (labile and refractory)) are represented. The soil nitrogen transformations include plant uptake of nitrate and ammonium, return of plant nitrogen to organic nitrogen, denitrification or reduction of nitrate-nitrite, immobilization of nitrate-nitrite and ammonium, mineralization of organic nitrogen, fixation of atmospheric nitrogen, volatilization of ammonium, and the adsorption/desorption of ammonium and the organic forms.

Two options are available to solve the transport equations: (1) the original backwards-difference implicit scheme that may be affected by excessive numerical dispersion at high Peclet numbers, or (2) the method of characteristics algorithm that eliminates numerical dispersion while slightly increasing model execution time.

PRZM has the capability to simulate multiple zones. This allows PRZM and VADOFT to combine different root zone and vadose zone characteristics into a single simulation. Zones can be visualized as multiple land segments joined together in a horizontal manner. There are three reasons a user may choose for implementing multiple zones:

  • simulate heterogenous PRZM root zones with a homogeneous vadose zone
  • simulate a homogeneous root zone with heterogenous vadose zones
  • simulate multiple homogeneous root zones with multiple homogeneous vadose zones

Another feature for pesticide simulation is the ability to simulate as many as three chemicals simultaneously as separate compounds or as a parent-daughter relationship. This gives the user the option to observe the effects of multiple chemicals without making additional runs or the ability to enter a mass transformation factor from a parent chemical to one or two daughter products.

Predictions are made on a daily basis. Output can be summarized for a daily, monthly, or annual period. Daily time series values of various fluxes or storages can be written to sequential files during program execution for subsequent analysis.

Download PRZM3 (via CEAM at the EPA)