In large routing units with a time of concentration greater than 1 day, only a portion of the surface runoff will reach the main channel on the day it is generated. SWAT+ incorporates a surface runoff storage feature to lag a portion of the surface runoff release to the main channel.

This parameter controls the fraction of the total available water that will be allowed to enter the reach on any one day. For a given time of concentration, as surq_lag decreases in value more water is held in storage. The delay in release of surface runoff will smooth the streamflow hydrograph simulated in the reach.

All soils in the watershed will be initialized to the same fraction. If sw_init = 0.0, the model will calculate it as a function of average annual precipitation.

Please refer to the Theoretical Documentation for a description of the use of this parameter in the mineralization calculations.

This parameter controls the amount of nitrate removed from the surface layer in runoff relative to the amount removed via percolation.

This parameter controls plant uptake of phosphorus from the different soil horizons in the same way that n_uptake controls nitrogen uptake.

Phosphorus removed from the soil by plants is taken from the solution phosphorus pool. The importance of the phosphorus uptake distribution parameter lies in its control over the maximum amount of solution P removed from the upper layers. Because the top 10 mm of the soil profile interacts with surface runoff, the phosphorus uptake distribution parameter will influence the amount of labile phosphorus available for transport in surface runoff. The model allows lower layers in the root zone to fully compensate for lack of solution P in the upper layers, so there should not be significant changes in phosphorus stress with variation in the value used for p_uptake.

The phosphorus percolation coefficient is the ratio of the solution phosphorus concentration in the top 10 mm of soil to the concentration of phosphorus in the percolate.

Sediment routing is a function of peak flow rate and mean daily flow. Because SWAT originally could not directly calculate the sub-daily hydrograph due to the use of precipitation summarized on a daily basis, this variable was incorporated to allow adjustment for the effect of the peak flow rate on sediment routing. This factor is used in the MUSLE equation and impacts the amount of erosion generated in the HRUs.

The fraction of residue that will decompose in a day assuming optimal moisture, temperature, C:N ratio, and C:P ratio.

Sediment routing is a function of peak flow rate and mean daily flow. Because SWAT originally could not directly calculate the sub-daily hydrograph, this variable was incorporated to allow adjustment for the effect of the peak flow rate on sediment routing. This variable impacts channel degradation.

This parameter controls the amount of pesticide removed from the surface layer in runoff and lateral flow relative to the amount removed via percolation. This parameter is only important if pesticide transport is simulated.

Many studies have shown that after an application of soluble P fertilizer, solution P concentration decreases rapidly with time due to reaction with the soil. This initial "fast" reaction is followed by a much slower decrease in solution P that may continue for several years (Barrow and Shaw, 1975; Munns and Fox, 1976; Rajan and Fox, 1972; Sharpley, 1982). In order to account for the initial rapid decrease in solution P, SWAT+ assumes a rapid equilibrium exists between solution P and an "active" mineral pool. The subsequent slow reaction is simulated by the slow equilibrium assumed to exist between the "active" and "stable" mineral pools. The algorithms governing movement of inorganic phosphorus between these three pools are taken from Jones et al. (1984).

Equilibration between the solution and active mineral pool is governed by the phosphorus availability index. This index specifies the fraction of fertilizer P which is in solution after an incubation period, i.e. after the rapid reaction period.

A number of methods have been developed to measure the phosphorus availability index. Jones et al. (1984) recommends a method outlined by Sharpley et al. (1984) in which various amounts of phosphorus are added in solution to the soil as K2HPO4. The soil is wetted to field capacity and then dried slowly at 25°C. When dry, the soil is rewetted with deionized water. The soil is exposed to several wetting and drying cycles over a 6-month incubation period. At the end of the incubation period, solution phosphorus is determined by extraction with anion exchange resin.

The P availability index is then calculated as:

where pai is the phosphorus availability index, $P_{solution,f}$ is the amount of phosphorus in solution after fertilization and incubation, $P_{solution,i}$ is the amount of phosphorus in solution before fertilization, and $fert_{minP}$ is the amount of soluble P fertilizer added to the sample.

References

Barrow, N.J. and T.C. Shaw. 1975. The slow reactions between soil and anions. 2. Effect of time and temperature on the decrease in phosphate concentration in soil solution. Soil Science 119(2): 167-177. Link

Jones et al. 1984

Munns and Fox. 1976

Rajan and Fox. 1972

Sharpley. 1982

Sharpley et al. 1984

Root density is greatest near the surface, and plant nitrogen uptake in the upper portion of the soil will be greater than in the lower portion. The depth distribution of nitrogen uptake is controlled by n_uptake, the nitrogen uptake distribution parameter.

The importance of the nitrogen uptake distribution parameter lies in its control over the maximum amount of nitrate removed from the upper layers. Because the top 10 mm of the soil profile interacts with surface runoff, the nitrogen uptake distribution parameter will influence the amount of nitrate available for transport in surface runoff. The model allows lower layers in the root zone to fully compensate for lack of nitrate in the upper layers, so there should not be significant changes in nitrogen stress with variation in the value used for n_uptake.

The weighting factor is a function of the wedge storage. This parameter is only important if channel routing is simulated using the Muskingum routing method.

The evaporation coefficient is a calibration parameter for the user that was created to allow reach evaporation to be dampened in arid regions. The original equation tends to overestimate evaporation in these areas.

The phosphorus soil partitioning coefficient p_soil is the ratio of the soluble phosphorus concentration in the top 10 mm of soil to the concentration of soluble phosphorus in surface runoff.

The primary mechanism of phosphorus movement in the soil is by diffusion. Diffusion is the migration of ions over small distances (1-2 mm) in the soil solution in response to a concentration gradient. Due to the low mobility of solution phosphorus, surface runoff will only partially interact with the solution P stored in the top 10 mm of soil.

Normal flow is defined as the streamflow when the channel is at 0.1*bankfull depth. This parameter is only important if channel routing is simulated using the Muskingum routing method.

The maximum day length minus dorm_hr is equal to when dormancy occurs.

This parameter defines the fraction of field capacity water content above which denitrification takes place. Denitrification is the bacterial reduction of nitrate (NO3) to N2 or N2O gases under anaerobic (reduced) conditions. Because SWAT+ does not track the redox status of the soil layers, the presence of anaerobic conditions in a soil layer is defined by this variable. If the soil water content calculated as a fraction of field capacity is ≥ denit_frac, then anaerobic conditions are assumed to be present and denitrification is modeled. If the soil water content calculated as a fraction of field capacity is < denit_frac, then aerobic conditions are assumed to be present and denitrification is not modeled.

Normal flow is defined as the streamflow when the channel is at bankfull depth. This parameter is only important if channel routing is simulated using the Muskingum routing method.

Multiplier to USLE_K for soils susceptible to rill erosion.

This parameter is only relevant when using Green & Ampt.

This coefficient allows the user to control the rate of denitrification.

A positive value denotes an increase in precipitation with an increase in elevation while a negative value denotes a decrease in precipitation with an increase in elevation.

A positive value denotes a decrease in temperature with an increase in elevation.

This parameter is required if = 1.