 What is Evapotranspiration (ET)?
Wikipedia defines Evapotranspiration (ET) as the sum of evaporation and plant transpiration. Evaporation accounts for the movement of water to the air from sources such as the soil and bodies of water. Transpiration accounts for the movement of water within a plant and the subsequent loss of water as vapor through stomata in its leaves. Evapotranspiration is an important part of the water cycle.
This simplified schematic of the water cycle shows that evaporation removes water from water bodies and soils. Transpiration removes water from plants, such as crops, lawns, and trees.
Why is this Important?
Water is a scarce resource. Though South Jersey receives on average over 40 inches of rainfall per year, this rainfall does not always occur when our lawns need it most. So we irrigate our lawn to correct this deficiency.
But how much to irrigate and when? We want to add enough water to meet the plant’s needs, but not too much. Excessive irrigation depletes aquifers, contributes to non-point (runoff) pollution, and possibly pollutes the aquifers. Our lawns, on average, receive twice as much water through irrigation than what they need. How often do we see sprinklers running in a heavy rainstorm?
 One widely used method to estimate irrigation needs is to use Evapotranspiration (ET). Using a sophisticated weather recorder, we determine the amount of water our lawn loses daily. This is ET. We then sum up daily ET’s for three or four days depending on soil water holding capacity. Then Rainfall for the same period is subtracted from the 3 or 4 day ET total. This is the ET Index. If the ET Index is positive, we then water our lawn that amount.
How do I use ET to meet my lawn needs?
The South Jersey RC&D Council has partnered with United Water to provide the Daily ET Index for four locations: Toms River, New Jersey; Haworth, New Jersey; Wilmington, Delaware; and West Nyack, New York. This daily ET Index simplifies irrigation scheduling. All a homeowner needs to know is that they should consider watering every three to four days. (Use a three day cycle for Toms River, and a four day cycle for Haworth, Wilmington, and West Nyack.) On your irrigation day go to the webpage, newspaper, or other media outlet for your area and water accordingly.
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One of the most critical issues facing South Jersey is a dwindling water supply. In the past twenty years the population of South Jersey increased by 22%. At the same time irrigated cropland acreage has increased by over 10%. With this increased demand for water, New Jersey has declared two Critical Water Supply areas (map at right). 
instructions. 
