Model Calibration and Validation

The model is calibrated and validated with soil ECe data obtained by Champion et al. [14] at the Fruita Research Center in Grand Valley in Coloroda. The soil in the Center contains gypsum and soluble salts. The topsoil thickness is 74 cm of loam and sandy loam. The underlying material to a depth of 150 cm is stratified loamy fine sand, silt loam, silty clay loam and very fine sandy loam. The experimental site contained six lysimeters of 1.55 x 1.22 x 1.22 m deep. The weather data consists of daily maximum, minimum temperatures, solar radiation, wind speed, and precipitation. The soil data consists of soil moisture and ECe and the irrigation data contained the dates and rates of applied water. The lysimeters were surface irrigated with water from Colorado River having an average ECe of 0.65 dS/m. Alfalfa was grown in the lysimeters. The lysimeters were applied a total of 796 mm irrigation water from April 15 to September 23, 1986.

Data from one of the lysimeters is used for the model calibration. Table 22.1 shows the measured versus predicted ECe values in time along a soil depth. The computed mean absolute error (MAE) and mean relative error (MRE) for the results in Table 22.1 are, respectively, 1.17 dS/m and 34.9%. Table 22.2 shows the calibrated values of the model parameters that resulted in the simulations summarized in Table 22.1. The calibrated model was then applied to simulate salt variation in a soil column experiment of the same lysimeter in Fruita Research Center in

Table 22.1 Measured versus predicted ECe values as a result of the calibration procedure

Soil layer

Measured

Predicted

depth (cm)

ECe (dS/m)

ECe (dS/m)

Date

1.17

30.0

1.00

May 23,

1.69

60.0

4.26

1986

7.12

90.0

6.23

1.20

30.0

1.00

June 12,

1.72

60.0

3.25

1986

7.05

90.0

6.75

1.38

30.0

1.50

September

1.72

60.0

3.30

9, 1986

6.55

90.0

3.35

Table 22.2 Parameter values as a result of the calibration procedure

Soil layer depth (cm)

Parameter

0-30

30-60

60-90

Wilting point water content

0.116

0.120

0.059

Field capacity

0.398

0.418

0.359

Saturated conductivity (mm/h)

2.500

5.500

8.200

Initial gypsum concentration (mg/L)

1.100

4.600

8.800

Initial measured ECe (dS/m)

8.850

8.850

8.850

Table 22.3 Measured versus predicted ECe values as a result of the validation procedure

Measured

Predicted

Date

ECe (dS/m)

ECe (dS/m)

March 10, 1988

3.05

3.05

March 25, 1988

4.60

4.70

April 10, 1988

4.82

4.65

April 25, 1988

5.00

4.75

May 10, 1988

5.70

5.10

September 30, 1988

3.25

4.85

1988. Alfalfa in the same lysimeter received about 841 mm irrigation rate from April 20 to September 16, 1988. Table 22.3 shows the validation model results in predicting ECe in time in the 90 cm soil profile. The computed MAE=0.45 dS/m and MAE = 11.7 implying satisfactory performance of the model.

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