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Theory

Example of input soil file for use in Soil Utility

 

Theory

DRAINMOD requires the following soil dependent data:

 

 

The soil water characteristic is usually determined by standard laboratory procedures made on soil cores. The lateral conductivity has been found to be an important factor in the overall water balance.  It is site specific and should be determined by field measurement techniques such as the Auger Hole method.  The infiltration parameters can be estimated using the methods described in the DRAINMOD users manual or with the soil preparation program.

 

The volume drained versus water table depth table, is for 'drained to equilibrium' conditions (steady state).  The soil preparation program uses the soil water characteristic of each layer of the soil to produce 'volume drained' values for water table positions ranging from the surface to the bottom of the soil profile.  The soil structure in the plow layer is dependant on crop and tillage practices which may cause that layer to have a relatively high porosity.  This porosity would be calculated if soil water characteristic data are obtained from samples that include the macropores caused by plant roots and other biological activity. If the plow layer is not sampled, then the volume drained from the top 15-30 cm may be lower. The extra pore volume could be added, but with caution.  The slope of the drainage volume-water table depth  relationship is the drainable porosity.  Therefore the relationship can be approximated by relatively simple methods if the drainable porosity is known.

 

The soil water available to the plant is limited by the upward flux from the water table to the plant roots.  The soil preparation program includes a routine which calculates the maximum water table depth that will support a given upward flux value.  The program inputs are; average depth of the root zone (which must be less than the depth of the top layer for this program), depth of each layer, the maximum tension in the root zone when it is dry (usually not a sensitive parameter), and each layer's unsaturated conductivity vs. tension relationship.

 

Because the unsaturated conductivity is often not known, the soil preparation program includes a routine to obtain estimates of the unsaturated conductivity using the Millington and Quirk procedure.  There are alternate methods which may be used as well, such as Brooks and Corey's graphical procedure or Rawls and Brackensiek's estimates from soil texture.  Otto Baumer of the USDA-SCS has developed a method of predicting K(h) from soil texture.  He has programmed this method for DRAINMOD.  For any of these methods, it is important to remember that vertical conductivities should be used, not lateral.  Typically, vertical hydraulic conductivities are 1/2 to 1/4 of lateral conductivities.

 

A Note on determining the unsaturated conductivities:

 

 Soil hysteresis makes the relationship between conductivity and soil tension nonsingular; i.e. the conductivity at a given tension can assume any value between the two extremes shown by the drainage and the imbibition branches of the Kh curve.  The actual value will depend on antecedent moisture conditions.  Upward flux during the day is a drainage or drying process while the soil moisture is being depleted and it may be an imbibition process during the night while the root zone recharges.  Average conductivity values should be used if possible.

 

Generally, unsaturated conductivity curves, either in the literature or predicted by the methods above, are for the drainage branch of the curve and using them may overestimate the daily upward flux.  To adjust for this, it has been our experience that using conductivity values which are 1/2 of the drainage curve values provides conservative upward flux estimates.  (If using MILNQRK or Brooks and Corey, just make the saturated conductivities 1/2 of the measured values).

 

The necessary inputs to DRAINMOD are stored in a soil file (extension .SIN) and can be checked with graphs of the output of WTVOLDRN and UPFLUX.  DRAINMOD uses straight line interpolation between points.  Generally, 15 points are sufficient.

 

Description of the Soilprep Program:

 

The soil preparation program is composed of several routines which can be used to prepare soil dependent input data sets for DRAINMOD.  These routines are:

 

·        VOLDRN - a program to prepare volume drained versus water table depth table from the soil water characteristic curves of each layer in the soil.

 

·        MILNQRK -  a program to approximate the unsaturated conductivity of each layer in the soil using the soil water characteristic and saturated vertical conductivity. The method used is from Millington and Quirk.

 

·        UPFLUX -  a program to calculate steady state upward flux versus water table depth curve from the unsaturated conductivity function of each soil layer.

 

·        GRNAMPT - a program to produce Green and Ampt parameters versus water table depth.

 

The outputs from VOLDRN and UPFLUX can be graphed and any adjustments to DRAINMOD inputs can be done.  This allows a good visual verification of the results.  This is encouraged to avoid the error of assuming that results from these programs are absolutely correct.  They are not.  They make reasonable approximations using the input data provided, which may not be correct.  If field observations do not agree with the modeled predictions then most likely the field observations are better and should be used.

 

Example input for Soil Utility

 

An Example Input Data Set For Gilfords FSL

 

4 LAYERGILFORD FSAL, INDIANA, P. 49      1) Title of soil file

04                                       2) Total number of layers

09        2.0       20.0                 3) Number of water content/tension input for layer, saturated conductivity (cm/hr), bottom depth of layer (cm)

0.459               0.0                  4) Volumetric water content and tension input (cm), including 15000cm tension measurement / approximation                 

0.355               50.0                                           

0.303               100.0                                          

0.244               200.0                                          

0.214               330.0                                          

0.204               500.0                                          

0.194               1000.0                                         

0.142               5000.0                                         

0.130               15000.0                                        

11        0.20      30.0                 3) Number of water content/tension input for layer, saturated conductivity (cm/hr), bottom depth of layer (cm)

0.339               0.0                  4) Volumetric water content and tension input (cm), including 15000cm tension measurement / approximation                 

0.310               30.0

0.285               50.0                                           

0.246               100.0

0.223               150.0            

0.197               200.0                                          

0.176               330.0                                          

0.166               500.0                                          

0.163               1000.0                                         

0.114               5000.0                                         

0.082               15000.0                                        

08        0.20      70.0                 3) Number of water content/tension input for layer, saturated conductivity (cm/hr), bottom depth of layer (cm)

0.367               0.0                  4) Volumetric water content and tension input (cm), including 15000cm tension measurement / approximation                    

0.284               50.0                                           

0.229               100.0                                          

0.157               200.0                                          

0.109               500.0                                          

0.105               1000.0                                         

0.062               5000.0                                         

0.047               15000.0                                        

09        0.20      500.0                3) Number of water content/tension input for layer, saturated conductivity (cm/hr), bottom depth of layer (cm)

0.386               0.0                  4) Volumetric water content and tension input (cm), including 15000cm tension measurement / approximation                   

0.251               50.0                                           

0.185               100.0                                          

0.111               200.0                                   

0.080               330.0                                          

0.066               500.0                                          

0.049               1000.0                                         

0.022               5000.0                                         

0.013               15000.0                                        

45, 30,  1                               5) Maximum root depth (cm) for Green-Ampt calculations, root depth (cm), layer of the soil water characteristic to use for DRAINMOD's maximum root depths

 

 

 

NOTES

1) Title

 

2) Number of layers

 

3) Number of inputs for the layer, saturated conductivity in cm/hr, depth of at the bottom of the layer in cm

 

4) Volumetric water content and tension (cm), including a 15000 cm for each layer

 

5) Maximum root depth in cm for the Green-Ampt calculations, root depth in cm, and the layer of the soil water characteristic to use for DRAINMOD's maximum root depths

 

A file similar to the example is the only one required for all programs.  Any other intermediate files are created and deleted when the program finishes.