WINDS Salinity and Nitrogen Algorithms

Abstract

This chapter introduces the WINDS (Water-use, Irrigation, Nitrogen, Drainage, and Salinity) model, which simulates water, salts, and nitrate in agricultural fields. The WINDS model uses daily time steps and allows for up to 13 soil layers and multiple field positions in simulations. The focus of this chapter is the nitrogen and salinity models in WINDS. The input salinity parameters include initial concentration in each soil layer, date of manure application, manure salinity, crop salinity equation threshold and slope, and irrigation water salinity. Unlike the salinity model, the nitrate model includes reaction terms: mineralization, denitrification, and plant uptake. Because nitrate is the primary form of nitrogen used by plants, the mass balance for nitrogen focuses on the sources and sinks for nitrate. The rate of microbial activity (mineralization and denitrification) is strongly dependent on temperature; thus, this chapter presents algorithms for calculating annual and diurnal temperature variation in soils. The input nitrate parameters include initial concentration in layers (mg/kg), organic matter distribution with depth, fertilization application dates and depths, and irrigation water nitrate concentration. Reaction terms include mineralization rate constants, Michaelis-Menton uptake coefficients, seasonal nitrate requirement, optimal soil nitrate concentration, fraction of plant nitrogen uptake as nitrate, nitrogen dissolution rate, and nitrogen stress factors.

Questions

• 1. An application of 100 kg/ha of NO₃-N is incorporated to a depth of 0.15 m. The 100 refers to only the N portion of nitrate. All of the fertilizer is dissolved on the day that it is incorporated. Calculate the change in the concentration of fertilizer in the upper 0.15 m per soil volume and in soil water. Water content is 0.3 L/L.

• 2. Annual Tucson temperature data at 10 cm and 50 cm depth is available in the Chapter 25 WINDS salinity and nitrogen workbook. Develop an annual sin wave Eq. (25.16) based on this data, and plot temperatures at 5, 10, 50, and 100 cm.

• 3. Hourly data is available in the Chapter 25 WINDS salinity and nitrogen workbook for 3 days in January and 2 days in June. Develop a diurnal sin wave Eq. (25.16) based on this data, and plot temperatures at the surface and at 5, 10, 50, and 100 cm for January and June. Compare the lag times calculated with the diffusivity to the lag times observed in the figures. Compare the equations that were derived based on data for the two different seasons.

• 4. Find the mineralization over a 2 day period in a sandy loam soil with θ_pwp θ_low, θ_high, and θ_sat are equal to 0.1, 0.17, 0.26, and 0.42, respectively. Find the final concentration of nitrate in soil water if the initial concentrations are 15, 8, and 3 mg/L in layers 1, 2, and 3 respectively assuming no water content changes and no other sources or sinks of nitrate. Surface organic matter content is 2,000 μg/g, K_mnl = 0.00005 d⁻¹, α = 0.021. The average water contents and temperatures in the top three layers are

  Water content	Cell elevations		Temperature
  Layer 1	0.16	0–40 cm,	27 °C
  Layer 2	0.28	40–80 cm,	22 °C
  Layer 3	0.25	80–120 cm,	20 °C

• 5. Calculate the denitrification rate in the sandy loam soil described in question 14 at 60 cm depth. Assume that initial nitrate concentration in soil water is 8 mg/L, and that the denitrification rate constant is 0.002 d⁻¹. Let the depth adjustment factor equal 0.021. Calculate the change in concentration within the soil volume and change in concentration within the soil water. Consider both the denitrification and mineralization to calculate the final concentration in layer 2 after 2 days.

• 6. Determine the seasonal removal of nitrate from the root zone (mg/L_soil) for a crop that has a yield of 8,000 kg/ha and has a nitrogen percentage of 1.6 %. Root zone depth is 1.5 m.

• 7. N_req = 1.4 kg/ha, N_min = 2 mg/kg soil, and N_max = 1.8 kg/ha. The fraction of nitrogen taken up as nitrate is 1.0. The optimal level of nitrate in the soil is 30 mg/kg. Find Km, plot the uptake rate vs. soil nitrogen concentration, and calculate the uptake of nitrate at a soil nitrate concentration of 20 mg/kg.