Water and Solute Mass Balance Models

Abstract

This chapter describes the WINDS tipping bucket approach to water, salt, and nitrate transport in soils. The tipping bucket model is based on the law of conservation of mass (Fig. 26.1). The model runs quickly because rapid changes due to irrigation or storm events are simulated with conservation of mass, which allows for daily time steps. The algorithms march forward in time with Euler’s finite difference method. Because the model is fast, it can simulate daily changes in water, salinity, and nitrogen at hundreds of locations within fields over a growing season.

Questions

• 1. Repeat Example 26.1, but change the infiltration from the first storm to 3 cm, and the field capacity to 0.25. As before, infiltration from the second storm is 4 cm.

• 2. Redo question 1, but divide the soil into three layers of 0.4 m depth.

• 3. Redo Example 26.3, but change the upper layer FC is 0.26, and the lower layer FC to 0.24. Also, 70 % of ET is removed from the upper layer and 30 % from the lower layer.

• 4. Redo Example 26.4 with the WINDS model and by hand, but lower the leaching fraction to 0.05. Make calculations for the irrigation on the third day for the upper two layers by hand. Next, use the WINDS model to calculate EC for 100 days. There are only two field sections in the WINDS Chapter 26 workbook. The sections are organized with respect to their irrigation zones in the spatial data worksheet. Add another G01 section in column C and write “3” in the same row in column. In cell K7, specify that the number of cells is 3 and click the Make new sections button. This process adds the C_3 worksheet to the end of the workbook. The next step is to populate the date in the Crop_data worksheet for section 3. You can do this in the Active Data worksheet or just copy the cells from section 2 (column C) to section 3 (column D) in the Crop_data worksheet. If you use the Active_data worksheet, then the copy the information from section 2, “Copy data from crop data,” and then copy rows 3 to 450 to section 3 (specified in cells G13:G16) and click the “Copy data to crop data” button. After calculating the required application depth for 0.05 leaching fraction, add the calculated fraction of baseline irrigation to the section 3 column in the G01 worksheet. Go to the Main worksheet. In cell G2, specify that three sections will be evaluated. After clicking Run, select position 3 in the Get Data combo box (upper right side of the worksheet). Find the “Water content” graph and the “Irrigation, rain depth, and leaching” graph with the Selection form. If rainfall appears in the graph, remove the rainfall from the Active year weather page for the first 100 days. Find the soil water salinity graph in the Salinity worksheet. Compare to the salinity levels in Example 26.4. Copy and paste the worksheets or graphs into this document. Use the graphs to assess the processes.

• 5. Calculate leaching fraction for irrigation water salinity 2 dS/m and required EC_e 1.5 dS/m?

• 6. What are the ratios EC_e/EC_ave, and EC_e/ EC_dw in Example 26.4? EC_dw is the leachate salinity. Discuss the importance of understanding these ratios with respect to crop management decisions?

• 7. Redo Example 26.5, but change the fertilizer application to 40 kg/ha on the first day application and change nitrate concentration in the irrigation water to 20 mg/L. Make a new hand calculation of the changes due to fertilizer application and irrigation during the first 3 days in the upper cell. Run the WINDS simulation for 100 days with the higher irrigation water nitrate concentration and higher fertilization rate on day 3. The irrigation rate will be the same as question 4. You can change the nitrogen data in the Active_data worksheet and copy it to section 3 in the Crop_data worksheet. Make sure that cell G5 in Main worksheet is marked True. Run the simulation from the Main worksheet. Select 3 in the Get_data combobox in the Main worksheet. Click the View Nitrogen data button. Copy the following graphs into your homework document: Nitrate (mg/kg) in layers, Irrigation and drainage nitrate (you might need to update both x and y axes from the selection form or from the axes), Reactions, and Cumulative leaching, nitrate and reactions. Assess the processes by looking at the graphs.

• 8. A soil has three 0.4 m layers, numbered 1, 2, and 3 from the bottom, with field capacity in all layers equal to 0.25. The initial water salinity in layers 1, 2, and 3 is, 23-, 7-, and 5-dS/m, respectively. ET is 10 mm/day with 20 %, 30 %, and 50 % of ET in layers 1, 2, and 3 respectively. Irrigation water salinity is 2 dS/m. The initial water content on the previous day in layers 1, 2, and 3 is 0.18, 0.15, and 0.10, respectively. Soil porosity is 0.4. An irrigation event adds 11 cm water to the soil in the morning. Compare to the final water content, actual salinity, and saturated paste extract salinity before the morning irrigation event. Compare the changes in water salinity and saturated paste extract salinity.

• 9. During a one day period, the upper layer of soil, 0.4 m depth, has a mineralization rate of 0.1 mg/L * m, a denitrification rate of 0.05 mg/L*m, and plant uptake of 1 kg/ha. One cm (average for the field) depth of water is added to the layer by drip irrigation and the irrigation water has a nitrate concentration of 20 mg/L. Transpiration removes 1.4 cm from the layer. No water leaches to the next layer. The initial water content is 0.18, and the initial nitrate concentration in the soil water is 15 mg/L. Calculate the final water content and nitrate concentration in the water. Calculate the kg/ha nitrate in the layer at the end of the day.