Chapter 17 SPAW Model -- Saxton & Willey Page: 1
THE SPAW MODEL FOR AGRICULTURAL FIELD AND POND
HYDROLOGIC SIMULATION
Dr. Keith E. Saxton and Mr. Patrick H. Willey
OUTLINE
INTRODUCTION
HYDROLOGIC SYSTEMS AND PROCESSES
FIELD HYDROLOGY
POND HYDROLOGY
EXAMPLE APPLICATIONS
FIELD DATA AND METHODS
FIELD DATA
Climatic Data
Soil Profile
Crop Growth
Crop Management
FIELD METHODS
Runoff
Infiltration
Potential ET
Evapotranspiration
Soil Water Redistribution
POND DATA AND METHODS
PHYSICAL DESCRIPTION
SINKS AND SOURCES
SIMULATIONS AND RESULTS
CALIBRATION AND SENSITIVITY
CORROBORATION OF RESULTS
EXAMPLE WATER BUDGETS
DRYLAND CROPPING
IRRIGATED CROPPING
WASTEWATER STORAGE POND
WETLAND HYDROLOGY
OPTIONS AND ENHANCEMENTS
CURRENT MODEL STATUS
INSTALLATION AND AVAILABILITY
SUMMARY
REFERENCES
Chapter 17 SPAW Model -- Saxton & Willey Page: 2
THE SPAW MODEL FOR AGRICULTURAL FIELD AND POND
HYDROLOGIC SIMULATION
Dr. Keith E. Saxton
1
and Mr. Patrick H. Willey
2
INTRODUCTION
The SPAW (Soil-Plant-Air-Water) computer model simulates the daily hydrologic water budgets
of agricultural landscapes by two connected routines, one for farm fields and a second for
impoundments such as wetland ponds, lagoons or reservoirs. Climate, soil and vegetation data
files for field and pond projects are selected from those prepared and stored with a system of
interactive screens. Various combinations of the data files readily represent multiple landscape
and ponding variations.
Field hydrology is represented by: 1.) daily climatic descriptions of rainfall, temperature and
evaporation; 2.) a soil profile of interacting layers each with unique water holding characteristics;
3.) annual crop growth with management options for rotations, irrigation and fertilization. The
simulation estimates a daily vertical, one-dimensional water budget depth of all major hydrologic
processes such as runoff, infiltration, evapotranspiration, soil water profiles and percolation.
Water volumes are estimated by budget depths times the associated field area.
Pond hydrology simulations provide water budgets by multiple input and depletion processes for
impoundments which have agricultural fields or operations as their water source. Data input and
selection of previously defined data files are by graphical screens with both tabular and graphical
results. Typical applications include analyses of wetland inundation duration and frequency,
wastewater storage designs, and reliability of water supply reservoirs.
The objective of the SPAW model was to understand and predict agricultural hydrology and its
interactions with soils and crop production without undue burden of computation time or input
details. This required continual vigilance of the many choices required for the representation of
each physical, chemical and biological process to achieve a “reasonable” and “balanced”
approximation of the real world with numerical solutions.
Over the development period, both the model and the method of data input with system
descriptors have evolved for improved accuracy, extended applications, and ease of use. The
program documentation includes theory, data requirements, example applications, and
1
Research Agricultural Engineer, USDA-ARS (Retired), Pullman, WA.
2
Drainage and Wetland Engineer, USDA-NRCS, National Water and Climate Center, Portland,
Oregon.
Chapter 17 SPAW Model -- Saxton & Willey Page: 3
operational details. The model results have been corroborated through research data, workshops
and application evaluations.
The SPAW-Field model is a daily vertical water budget of an agricultural field, provided the
field can be considered, for practical purposes, spatially uniform in soil, crop and climate. These
considerations will limit the definition of a “field” depending on the local conditions and the
intended simulation accuracy. For many typical cases, the simulation will represent a typical
farm field of tens to a few hundred acres growing a single crop with insignificant variations of
soil water characteristics or field management. In other cases, a single farm field may need to be
divided into separate simulation regions because of distinct and significant differences of soil or
crop characteristics. These definitions and divisions will depend on the accuracy required,
however users soon gain enough experience through alternative solutions to guide these choices.
Since the field model has no infiltration time distribution less than daily and no flow routing, it is
generally not applicable for large watershed hydrologic analyses. However, it can be utilized for
water budgets of agricultural watersheds composed of multiple farm fields, each simulated
separately and the results combined. The combined field concept to represent a watershed is
used as an input source for the pond simulations. With no streamflow routing there are no
channel descriptors included. Daily runoff is estimated as an equivalent depth over the
simulation field by the USDA/SCS Curve Number method.
The SPAW-Pond model simulates the water budget of an inundated depression or constructed
impoundment. The water supply to the inundated area is estimated runoff from one or more
previously simulated fields, plus, if applicable, that from external sources such as an off-site
pump or flush water from an animal housing facility. Pond climatic data are provided from that
input to the field simulation. Additional features are included such as outlet pipe discharge,
drawdown pumps, irrigation supply demands and water tables to allow for a wide variety of pond
situations described as wetlands, small ponds, water supply reservoirs, lagoons or seasonal
waterfowl ponds.
Basic interactions of soil chemicals such as nitrogen and salinity with soil water and crop
production are included. The chemistry is represented in daily budget form, thus does not
include interactions and minor processes which occur within soil and crop environments. These
budgets are useful as a screening tool to define potential effects and hazards related to the
chemical inputs and dispositions for situations often encountered in agricultural hydrologic
analyses.
HYDROLOGIC SYSTEMS AND PROCESSES
Simulating the hydrologic budget of an agricultural field or pond requires defining the
hydrologic system and associated processes. The field budget utilizes a one-dimensional vertical
system beginning above the plant canopy and proceeding downward through the soil profile a
depth sufficient to represent the complete root penetration and subsurface hydrologic processes
(lateral soil water flow is not simulated). The pond hydrologic system is an impoundment with
external inputs from a watershed or supplemental water sources and outflow by spillways, pumps
and seepage. The following schematics illustrate the field and pond hydrologic systems and
major processes.
Chapter 17 SPAW Model -- Saxton & Willey Page: 4
FIELD HYDROLOGY
The principle hydrologic processes in the SPAW-Field model are depicted in Figure 17.1 by a
schematic of the vertical budget of an agricultural field:
Precipitation : Daily observed totals, although snow accumulation and snowmelt are
estimated when air temperature is included. Applied irrigation water is a supplement without
runoff.
Runoff : Computed by the USDA/SCS Curve Number method as a percent of daily rainfall
from parameters of soil type, antecedent soil moisture, vegetation, surface conditions and
frozen soil. No stream routing is provided. Observed runoff can be substituted or compared
to simulated values.
Infiltration : A daily amount based on rainfall minus estimated runoff and stored in the
uppermost soil layers as available capacity permits.
Evapotranspiration : Combined daily estimates of plant transpiration, direct soil surface
evaporation and interception evaporation estimated from a daily atmospheric potential
evaporation reduced by the plant and soil water status. The potential evaporation input data
may be estimated by one of several methods such as the Penman and/or Monteith equation,
daily pan evaporation, temperature or radiation methods, or mean annual evaporation
distributed by months and monthly mean daily.
Redistribution within the soil profile: Infiltrated water is moved between soil layers by a
Darcy tension-conductivity method to provide both downward and upward flow estimates.
Soil water holding characteristics of tension and conductivity are estimated from soil textures
and organic matter and adjusted for density, gravel and salinity. Observed soil moisture can
be substituted or compared to simulated values.
Percolation : Water leaving the bottom layer of the described soil profile. Percolated water is
considered to be temporarily stored in an “image” layer just below the profile and is upward
retrievable. Upward percolation (negative) is considered for cases of groundwater
occurrence.
Deep drainage to groundwater or interflow occurs when the image layer achieves near
saturation and additional percolation occurs.
Chemical applications and redistributions : Nitrogen and salinity chemicals are budgeted
with plant uptake and soil water transport interactions to estimate soil layer quantities, soil
water concentrations and percolation.
Chapter 17 SPAW Model -- Saxton & Willey Page: 5
Figure 17.1: Hydrologic processes within the SPAW-Field system of an agricultural field.
POND HYDROLOGY
The principle hydrologic processes considered in the SPAW-Pond model are depicted in Figure
17.2 by a schematic of the inflows, withdrawals and losses. A depth-area table describes the
ponded area plus specific depths above the pond bottom for permanent storage, pump inlets,
pipe-outlet and the emergency spillway outlet. Each of these depths provides operational limits
of the various budgeting processes such as the pumps, pipe outlet, or irrigation water.
Watershed inflow : Daily water supplied to the pond by watershed runoff comprised of one
or more fields that have had runoff estimated by a SPAW-Field simulation.
Subsurface inflow : Daily water supplied to the pond by a percentage of the estimated
watershed field deep drainage.
Pond inflow: Outflows from an upstream pond estimated by a prior pond simulation.
Side Slope Runoff : Runoff from pond side-slopes above the current water level.
External input : Water supplied to the pond from a source other than a watershed such as an
off-stream pump or an animal housing flush system. An optional pump control by specified
upper and lower pond depth limits provides hydrologic-based decisions.
Rainfall: That precipitation falling directly on the pond water surface.
Evaporation : Daily water surface evaporation estimated as the potential of the climatic data.
