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Improving your irrigation

Thursday 13, Feb 2020

 

In his latest column, John Neylan focuses on irrigation management for sportsfields and asks sports turf managers whether they are making the best use of their most precious resource.

Producing quality turf areas depends on the availability of a good quality and constant water supply.  Water and how it is applied is a major factor in producing a high-quality turf.  Over the past 12 months there have been two particular aspects related to the irrigation of sportsfields that has come to my attention which has had me questioning some current techniques and strategies being employed. 

Irrigation scheduling and depth of water penetration

During the STA Victoria-funded sportsfield research project (the results from which were presented in my column in ATM Volumes 21.2 and 21.3), it was obvious over the summer months that some fields were not receiving sufficient water because the watering programme was locked into a time programming basis rather than plant water requirements. 

Because the past two irrigation seasons have extended into late April, the fields, albeit they were couch, were under severe moisture stress going into the autumn.  A search through Nearmap images confirms that this is a widespread problem.  From my research there are two components as to why the fields were under stress;

  • The poor water distribution of many irrigation systems; and 
  • The inadequate amount of water being applied. 

In measuring soil moisture on a regular basis, the soil moisture content started in the spring being at an adequate level and then steadily declined throughout the dry months.  Shallow irrigations were most noticeable and none of the fields at any stage were being irrigated to the full depth of the rootzone.  In fact, looking at the run times on many irrigation systems, they were locked into such short run times and struggled to apply 5mm of irrigation per irrigation cycle. 

Consequently, most of the available moisture was in the top 25mm of the profile where there were increased losses due to evaporation.  More importantly it this did not allow the deep root system of the warm-season grasses to be used to their full potential. 

Soil type and water conservation

With impending water restrictions in some states and the increasing cost of water, there has been an ongoing discussion regarding how water usage can be reduced on sportsfields.  One particular aspect that has intrigued me is the promotion of using fine-textured soils to reduce water use.  The premise being that fine-textured soils (i.e. not sands) have a greater water holding capacity and therefore reduce the water demand.  This theory is flawed and will be discussed below.

Water can be a high cost input in maintaining turf areas and in managing turf it is important to understand the role of water in grass growth.  Unless the turf manager understands where water fits into the turf management picture, then understanding water use efficiency is not possible.  In fact, a better understanding of water use efficiency can improve turf health, wear tolerance, wear recovery, while giving a better-quality surface as well as conserving water.

Managing irrigated turf requires knowledge of the grass species being grown and its water requirements, soil types and the effects of cultural techniques used to maintain them.  With most sportsfields being warm-season grasses and, in particular, couch, it is quite clear that many turf managers are not exploiting the characteristics of these grasses in terms of how irrigation is programmed.  Grasses such as couch have deep root systems that can exploit moisture at depth, but couch fields are rarely irrigated to the depth of the rootzone as a method of water conservation. 

Turfgrasses and Plant Water Use

The prime purpose of irrigating turf is to replace the water used by the plant.  There are two important criteria to understand when discussing plant water use – water use rate and water use efficiency. 

Water use rate (WUR) is the total water requirement for turfgrass growth plus the quantity transpired from the plant and evaporated from the soil surface (Youngner, 1985).  The WUR is typically measured as evapotranspiration and changes with climatic conditions, plant species (and cultivars), soil moisture availability and cultural practices. 

Water is a critical component of plant growth and the production of new tissue is necessary for the plant to recover from wear. Water is a key component of photosynthesis and the rate of photosynthesis is dependent on water availability as well as nutrient supply and the ability of the rootzone to extract these from the soil.  Only a small proportion (about two per cent) of water absorbed by a turfgrass plant is used in photosynthesis and growth processes.  The majority (the remaining 98 per cent) of the water is transpired from the turfgrass surfaces into the surrounding atmosphere. 

The total water loss from the turf sward is described as evapotranspiration (ET).  While there are generic figures for ET quoted in the literature, cultural or environmental factors that alter the leaf area or shoot density of a given species may result in a significant shift in its relative ranking compared to the other species.  Significant variability occurs among cultivars within the species. 

As ET is a measure of the irrigation requirements of a turf sward, then if water use efficiency is to be improved then what are the most desirable turf sward characteristics?  Beard (1989) described the most desirable sward as one which has a high shoot density, high leaf number, horizontal leaf orientation, narrow leaves and slow vertical leaf extension rate.  These criteria fit well with warm-season grasses.

Click here to read the full article, originally published in Volume 21.6 (Nov-Dec 2019) of the Australian Turfgrass Management Journal. 

Acknowledgements John Neylan is a turfgrass agronomist with Melbourne-based Sporteng. 

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