Three-block tomato schedule after a 28 mm storm
Field: 1 ha tomato on raised beds 1.2 m centre-to-centre, one row per bed, plants 0.4 m apart, one 16 mm lateral per bed with 2 lph drippers at 0.4 m. Application rate is about 4.2 mm/h. The field is divided into three valve blocks of about 0.33 ha each. Soil: medium black, field capacity 30% and permanent wilting point 16% by volume, effective root depth 30 cm, management allowed depletion 50%. Weather: ET0 6.5 mm/day, crop coefficient 0.85, no rain for 10 days. Current practice: the farmer opens one valve each morning for 2 hours, so each block is irrigated once every three days. Rain: on day 11, 28 mm falls in 40 minutes; the field has 0.5% slope and some runoff. Water and soil: borewell water EC 0.9 dS/m; soil saturation extract ECe 2.8 dS/m. The farmer says the crop is 'starving for water' because the top 3 cm is dry and wants to irrigate 3 hours per valve. A neighbour advises stopping irrigation for a week after the rain.
1. Calculate the average depth the current schedule applies and compare it with crop demand. Is the farmer under- or over-irrigating? What should he change?
Read the worked answer
Application rate = 2 lph / (1.2 m × 0.4 m) = 4.17 mm/h. Each block is 0.333 ha, so 2 h applies 4.17 × 2 = 8.3 mm per event. With one block every 3 days, average = 8.3/3 = 2.8 mm/day. Crop demand: ETc = 6.5 × 0.85 = 5.5 mm/day. Net requirement is 5.5 mm/day; at 90% application efficiency, gross = 6.1 mm/day, or 18.4 mm per 3-day cycle, which needs 18.4/4.17 = 4.4 h per block. The current 2 h supplies only about 45–50% of demand. The dry top 3 cm is not evidence of shortage: drip wets a bulb below the surface, and surface drying is normal. Increasing to 3 h helps but gives only 12.5 mm per event = 4.2 mm/day, about 76% of demand. The correct change is about 4.5 h per block every third day, or 1.5 h per block daily if the pump and power allow (3 blocks × 1.5 h = 4.5 h/day). Check soil moisture at 15–20 cm with an auger before changing, and measure emitter flow at the head and tail of each block.
2. How should the 28 mm rain change the schedule? How many days can the farmer safely skip, and what field check should he use before restarting?
Read the worked answer
Effective rain: with 0.5% slope and 28 mm in 40 minutes, runoff is likely; use 70–80% effectiveness, i.e. 20–22 mm. At 5.5 mm/day, that replaces 3.6–4.0 days of ETc. Do not stop for a week. Check the soil at 15–20 cm on days 12 and 13; if the ball test shows moisture below field capacity, restart on day 15, i.e. skip 3 days, with the revised 4.4–4.8 h per block. If the rain infiltrated fully with no runoff, skip 4 days. Because the soil is black and cracking, check whether the storm wetted only 10–15 cm while the root zone is 30 cm; the subsoil may still be dry. Use a tensiometer or a hand-feel test at 20 cm, not the surface. Do not restart with a full 4.4 h immediately after rain; split the first set to avoid leaching nitrate below the root zone. Record the rain date and the restart date in the irrigation log.
3. The soil ECe is 2.8 dS/m and the water EC is 0.9 dS/m. How does salinity change the runtime, and what should the farmer monitor?
Read the worked answer
Salinity matters. Tomato is moderately sensitive; ECe 2.8 dS/m is above the threshold of about 2.5 dS/m, so some yield loss is likely. Leaching requirement LF = ECw/(5ECe − ECw) = 0.9/(14.0 − 0.9) = 0.069, about 7%. Gross requirement becomes 6.1/(1 − 0.07) = 6.6 mm/day, and runtime 6.6/4.17 = 1.58 h/day equivalent, or 4.75 h per block every third day. Do not increase leaching by flooding; drip with a modest leaching fraction and monitor ECe each season. If ECe rises above 4 dS/m, yield loss becomes significant and a different strategy is needed: more frequent irrigation, gypsum if the soil is sodic, and better drainage. The borewell EC of 0.9 dS/m is moderate, not a reason to ignore filtration or salinity. Test the soil at the start and end of the season, and keep the fertigation EC below about 2.5 dS/m in the drip line.