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Criterion

Explanation

Developer

United States Department of Agriculture (USDA)

Hardware computing requirements

No hardware computing requirements specified.

Code language

FORTRAN 77

Original application

Predicts the salinity, sodicity, and toxic-solute concentration of the soil-water within a simulated crop rootzone resulting from the use of a particular irrigation water of a given composition and at a specified leaching fraction.

Used to evaluate how salinity (or solute concentration) affects crop yield, and how sodicity level affects soil permeability.

Can be compared with transient models including TETrans and UNSATCHEM

Non-computer version of the Model model can be used where computer facilities are lacking.

Public/proprietary and cost

Public; No cost

Physically or empirically based

Physically based

Mathematical methods used

Relatively simple, steady state model judges water suitability for irrigation under one meaningful, reference condition (i.e. steady-state – the likely worst-case situation with maximum build-up of salinity and sodicity.)

Major cation and anion concentrations are predicted at equilibrium as a function of irrigation water composition, leaching fraction, soil CaCO3 presence or absence, and several alternative amendment treatments.

Average soil water EC and SAR are also calculated for both the whole rootzone and upper one-half of the rootzone. The EC and Sodium absorption ratio (SAR) of the soil water at the top of the rootzone are given in the printout to aid in judging the likelihood of permeability and tilth problems.

Predicted soil water compositions, salinities and sodicties from Watsuit are compared against standards of acceptance with respect to salinity, permeability and crusting and toxicity criteria.

In frequent irrigation scenarios, the effect of salinity on crop yield is evaluated using either water-uptake-weighted EC (expressed in units of deciSiemen per meter (dS/m)) or osmotic potential.

Terms for predicted soil water composition at equilibrium, as well as water-uptake-weighted chloride concentration and osmotic potential are:
Ca⁺, Mg, Na C03^-, HCO3, Cl, SO4^-, pH, EC
for each of five relative soil depths – the soil surface, 1/4, 1/2, 3/4, and full depth of the rootzone.

Watsuit can calculate the relative magnitude to which salt precipitation and mineral weathering affect levels of soil water salinity depending upon irrigation water compositions and leaching fraction.

Input data requirements

Requires annual (or longer) averages of water consumption and leaching fraction.

Input composition of the irrigation water should contain equal equivalent concentrations (meq/l basis) of cations and anions. If not, they must be made equal. This is best done by someone knowledgeable of the chemistry of the water in question. If not made equal at input, the model invokes a subroutine to satisfy equivalency requirements.

Leaching fraction option must be selected at input.

At input, user must accept or reject the assumption of soil-lime saturation. Select amendment option(s):
(a) addition of sulphuric acid to the irrigation water to replace 90 percent of the alkalinity with sulphate (chemical equivalent basis),
(b) addition of gypsum to the irrigation water in amount equivalent to 1 or more mmol/L of CaSO4 to simulate water- or top-dressed soil-treatments with gypsum, or
(c) incorporation of gypsum in the soil in an amount that will add the equivalent of 20 mmol/L of Ca++ and S04- to the infiltrating water to simulate soil-incorporated treatment with a substantial amount of gypsum.

Outputs

Output file showing predictions

Can be printed to a disc or using DOS prompt PRINT filename (return)

Representation of uncertainty

Given the uncertainty of the precise threshold levels of soil water SAR and irrigation water EC, the SAR and EC for irrigation water are taken as generally suitable estimates of levels resulting in the surface soil for purposes of assessing permeability and tilth hazard. For highly sodic water, an adjusted SAR value is used in place of irrigation water SAR. (Non-computer version).

Prevalence

Used in studies for leaching requirement and salinity as pertaining to areas such as the South Delta and Imperial Valley, as well as the Pecos River

Ease of use for public entities

Easily accessible; can be obtained on a floppy disk from FAO or from the senior author or downloaded on line.

Ease of obtaining information and availability of technical support

Service provider contact information and email address is provided through the CEOS Land Surface Constellation Portal
Name: WALTER RUSSELL
Email: wrussell at ussl.ars.usda.gov

Source code availability

Not specified

Status of model development

Fully developed, ready for use

Challenges for Integration

Steady state model may not integrate well with transient models. Has been used to estimate soil salinity in the Delta but has not been calibrated for Delta soils.