Question

Imagine you are responsible for scheduling irrigation for a corn crop. Your management goal it to keep the corn wellwatered so that the evapotranspiration from the field is equal to the reference (or potential) evapotranspiration rate. Create a spreadsheet to perform a simple daily soil water balance for the field to determine the number of irrigation events and total irrigation amount required in the 120 -d growing season assuming: \- The reference evapotranspiration rate starts at $2.0\mathrm{mm}{\mathrm{d}}^{-1}$ on day 1 of the season and increases by $0.1\mathrm{mm}{\mathrm{d}}^{-1}$ each day until day 60, after which it decreases by $0.1\mathrm{mm}{\mathrm{d}}^{-1}$ each day. Rainfall of $12\mathrm{mm}$ occurs early in the morning on day 7, day 14, and every 7 th day after that for the duration of the season, and all the rain infiltrates into the soil. The root zone is $1-\mathrm{m}$ deep silt loam with a "field capacity" of $0.23{\mathrm{cm}}^3{\mathrm{cm}}^{-3}$ and permanent wilting point of $0.09{\mathrm{cm}}^3$ ${\mathrm{cm}}^{-3}$ and is at field capacity on day $0.$ \- Each irrigation event occurs on the last possible day to maintain FAW $>0.5$. The water added in each irrigation event is the amount required to restore the profile to field capacity based on the soil water storage at the end of the previous day.

Short answer

Question: Estimate the number of irrigation events and total irrigation amount required for a corn crop during the 120-day growing season based on the given information and the spreadsheet-based calculations.

Step-by-step solution

Setup the spreadsheet

Create a spreadsheet with rows representing days (1 to 120) and columns representing the following variables: - Day - Reference Evapotranspiration Rate (ETref) - Rainfall - Soil Water Content (SWC) - Fraction of Available Water (FAW) - Irrigation Event (Yes or No) - Irrigation Amount

Calculate ETref

Use the given information to calculate the reference evapotranspiration rate (ETref) for each day: - Day 1: 2.0 mm/d - Day 2-60: 2.0 + 0.1*(day-1) mm/d - Day 61-120: ETref(day-1) - 0.1 mm/d

Add rainfall data

Fill in the Rainfall column with the given data (12 mm on day 7, 14, and every 7th day after that).

Calculate Soil Water Content

For each day: - Calculate the change in soil water content (SWC) due to ETref and rainfall - Update SWC: SWC(day) = SWC(day-1) - ETref(day) + Rainfall(day) - Ensure SWC stays within the range [(Soil Depth * Permanent Wilting Point) and (Soil Depth * Field Capacity)].

Calculate Fraction of Available Water

Calculate FAW for each day using: FAW(day) = (SWC(day) - Soil Depth * Permanent Wilting Point) / (Soil Depth * (Field Capacity - Permanent Wilting Point))

Determine irrigation events and amount

For each day, check if an irrigation event is required: - If FAW(day) <= 0.5, then: - Irrigation Event = Yes - Irrigation Amount = (Soil Depth * Field Capacity) - SWC(day-1) - Else: - Irrigation Event = No - Irrigation Amount = 0

Calculate total irrigation events and amount

Count the number of irrigation events and sum the irrigation amounts in the respective columns to obtain the total number of irrigation events and the total irrigation amount required for the 120-day growing season. By completing these steps, you'll have a spreadsheet that performs a daily soil water balance, provides the number of irrigation events and total irrigation amount required for the corn crop during the 120-day growing season.