ET is not Extra-Terrestrial
By Colin Mumford
ET, the often quoted, but never quite explained. What is it? and should we be interested in it? Does it mean evergreen turf? electric trimmers?, elm trees? or extra-terrestrial? The answers to these and many more questions are deeply embedded in the soil-plant-water continuum, and this article aims to unravel a tiny bit of its mystery.
Turfgrass water use is related, amongst other things, to regulating the turfgrass' temperature. This requires the grass plant to lose water by what is essentially sweating, and is termed 'transpiration'. When this loss of water is combined with the loss of water from the soil - through evaporation - we arrive at the term 'evapotranspiration' or ET as most people refer to it. Amazingly ET can account for up to 80% of moisture loss from the soil, so by understanding and knowing your own particular turfgrass surfaces ET, you can tailor your irrigation practices to best suit the grass plant, and potentially reduce your water budget to meet the demands of current and future environmental regulation.
Many factors influence the rate of ET on your playing surface; these include the grass species, the height of cut, and the weather, which due to its constantly changing nature results in a continually changing ET rate. Consequently the rate of ET is likely to be different from one playing surface to the next. So how do you determine ET?
The methods employed for determining ET range from direct measurement with lysimeters, reference measurements with atmometers such as an evaporation pan or a porous porcelain plate, and empirical models that estimate ET based on weather data. But before we look at the different methods, a few terms need to be explained. Firstly, 'reference crop evapotranspiration' or ETo as it is referred to. This is a reference point that is used in determining the ET for all crops, and is a hypothetical crop that closely resembles an adequately watered grass sward with a uniform height of 0.12 m. The actual ET for the crop - turfgrass in this case - is referred to as ETc, and lastly, a 'crop coefficient' (Kc), which is the ratio between ETc and ETo i.e. the factor that ETo is multiplied by to determine ETc.
Ironically, many turfgrass practitioners already employ the use of a Kc in their irrigation scheduling, without even knowing it; this could be described as - in the words of Donald Rumsfeld - an "unknown known". What do I mean? With knowledge and experience of a given situation, many practitioners will adjust their irrigation practises, for example a particular area - say a shaded golf green - takes longer to dry out, so less water is applied, whereas the converse can apply to another green on the same site.
Measuring ET is quite straight forward, but can be time consuming, in fact it can take two to three years to acquire reasonable ETc and Kc values. The simplest method is to use a 'weighing lysimeter' which is essentially a receptacle with the same rootzone and turfgrass as the playing surface. The lysimeter is saturated and allowed to drain to field capacity, weighed to determine the initial weight, and then set into the ground, ideally the playing surface or adjacent to it in an area that has the same environment/maintenance regime as the actual playing surface. The lysimeter is then reweighed 24 hours later, and the ET can then be determined by calculating the difference between the initial weight and the final weight.
If you don't want to dig holes and insert lysimeters into your surfaces, you have the option of atmometers. These have the added advantage that they do not interfere with the playing surface, and can be dotted around the area of your facility to show the variation in ETo across the whole site, which will enable you to adjust your water applications accordingly. However, atmometers only generate the ETo value, ascertaining the Kc to calculate the ETc is more difficult as there has not been a great deal of research on sports turf ET in a UK environment. Finally, if you have a weather station on your site, or access to weather data from a nearby weather station, it is possible to estimate ETo values with an empirical model, such as the Penman-Monteith model. Fortunately computer software packages that carry out the calculations, once the weather data has been inputted, are available.
In an ever changing world where global warming and declining resources are at the forefront of European Union environmental policies, the correct management of irrigation water has become crucial. In other sectors of land management, for instance agriculture, the measurement or estimation of ET to aid irrigation scheduling is common practice. Perhaps now is the time to make it a common practice in sports turf management.
Colin has been working in the turf grass industry for well over 17 years. Back in 1999, after ten years in turfgrass management, Colin left his position of Head Greenkeeper at the North Weald Golf Club in Essex to become a full-time student. He is now in the final year of his engineering doctorate (EngD) at Cranfield University, where he is conducting research on precision irrigation for the Jockey Club.