At Pinney Purdue (PPAC) 1.66 inches of rain fell June 25 through July 11. The potential evapotranspiration (PET) over the period was 2.79 inches. Estimates for water use by the crops are lower than potential evapotranspiration because it is only 5 weeks after transplanting. For tomatoes, estimated water use was 2.01 inches, and for watermelons, 1.60 inches. The water use by tomatoes is not expected to equal the PET until next week. For watermelons, the maximum water use is expected in two weeks, when crop use is estimated to reach 90% of PET.
Both tomato and watermelon plants are in the early stages of first fruit growth. Plants are still actively increasing in size and setting more fruit. The earliest tomato fruit range in size from less than ½ inch to golf-ball size; watermelon fruit are from 1 to 5 inches long. Eggplant flowers are just starting to open. On pepper plants, the first fruit has just set on the largest plants. There are no clear differences in plant size, appearance, or fruit set in the different irrigation treatments. Watermelon plants in the unirrigated row look a little smaller, but they have appeared that beginning soon after transplanting, before irrigation began.
We installed a flow meter in the irrigation system last week. That gave us the ability to measure the amount of water applied, and to compare it to estimates based on the drip tape specifications. The drip tape specifications are 0.45 gallons per minute per 100 ft. at a pressure of 8 lbs. per square inch (PSI), or 27 gallons per hour (gph). We don’t have a pressure gauge in the drip tape itself, but there is a gauge downstream of the flow meter and just before the filter and automatic timer. During the few irrigations since the flow meter was installed, we have found that when the pressure gauge reads 15 PSI, we apply at least 25% more than the tape specifications at 8 PSI, or at least 34 gph per hundred feet. At a pressure of 10 PSI, we apply about 20% less, or 22 gph. When operating at 12 PSI, the gallons reported by the meter match the estimate based on the drip tape specifications, or 27 gph. The flow meter will help us apply the desired amount of water.
Figure 1. Unirrigated and irrigated tomatoes on July 12 in Pinney Purdue Ag Center irrigation demonstration (left) and flow meter installed in irrigation manifold (right).
More information about this demonstration can be found in the previous newsletter article
Funding for project Improve Drip Irrigation Management for Vegetables and Melon Production in Indiana was made possible by the Indiana State Department of Agriculture through grant A337-22-SCBG-21-003. Its contents are solely the responsibility of the authors and do not necessarily represent the official views of the ISDA.