Water Management in Turf Daniel C. Bowman

The early bird may get the worm, but it’s the second mouse that gets the cheese

Why Manage Water? It is only prudent that all turf managers assume a proactive posture and become good stewards of everyone’s water resources. If we learn how to effectively and intelligently manage our water supplies, there should be enough for everyone. What do we need to know to achieve this?

Background Americans use ~2000 gal/day, compared to 12 gal/day in undeveloped countries One egg uses 40 gal One ear of corn, 80 gal One pound of hamburger, 2500 gal One automobile, 100,000 gal

Background Americans use ~2000 gal/day, compared to 12 gal/day in undeveloped countries One egg uses 40 gal One ear of corn, 80 gal One pound of hamburger, 2500 gal One automobile, 100,000 gal

Background 97% of the world’s water supply is in the oceans 2% is in polar ice caps Only 1% of the total is fresh water!

Background Water is neither created or destroyed. Your cup of coffee this morning may have contained water molecules that Cleopatra bathed in. Given that, location and timing become crucial elements of water allocation.

What We Need To Know 1. How water behaves in different soils 2. How and why turf obtains water from the soil 3. How the plant uses and loses the water it obtains 4. How the manager can irrigate most efficiently to replace water lost

Water In The Soil Whether water is supplied through rainfall or irrigation, it can only be effective when it is able to infiltrate into the soil. Unfortunately, large amounts of water can be lost through surface runoff. How much depends on the type of soil, the topography, the moisture content, the precipitation or irrigation rate, and the presence of vegetation.

Water In The Soil It makes sense to try to minimize runoff losses by improving soil structure, contouring for gentle slopes, matching irrigation to infiltration, and maintaining a good turf cover.

Water In The Soil Once water has entered the soil, it tends to fill most empty pores, both macropores and micropores. Water continues to move downward, under the force of gravity, through the macropores. Eventually the macropores drain completely, and are refilled with air. Micropores, on the other hand, retain their water against the force of gravity. This is because water is both adhesive and cohesive.

Water In The Soil Micropores aren’t all one, uniform size. They range from relatively large to very tiny. The force with which water is held in the pores is related to the pore size. Larger pores have just enough force to hold the water against gravity’s force, but not enough force to resist the force of roots to obtain water. The small pores hold on to their water very tightly, much more tightly than the force of gravity, and often more tightly than the force of the root to extract water from the soil.

Saturation This means that only some of the micropores give up their water to the plant. Some retain their water even though the plant may be wilting from drought. We can thus identify several values with regards to soil moisture. The first is completely saturated, when all the pores are filled, as during a heavy rain. This may represent about 50% of the total soil volume

Field Capacity After drainage has removed the water from the macropores, the soil is at field capacity, with water occupying perhaps 30-35% of the total volume. But the soil doesn’t stay at field capacity very long.

Permanent Wilting Point Evaporation of water from the soil surface and absorption by plant roots start to deplete the water from the larger of the micropores, and the soil begins to dry out. After a while, the plant can no longer remove water from the smallest pores, and it starts to wilt. When the plant can no longer recover, even if irrigated, the soil is considered to be at the permanent wilting point, which may occur when soil water is around 10-15% of the total volume.

Available Water The difference between field capacity and permanent wilting point is the amount of available water. Take the case where field capacity is 35% and the permanent wilting point is 15%. The difference, 20%, is the amount of water, expressed as a percent of total volume, which is potentially available to the plant. We can use this to guestimate the amount of water available in a given rootzone.

Effective Available Water Water has to be both available to the plant, and in the rootzone, to be of any use to the plant. Water that percolates past the rootzone is lost to the plant, just as runoff is wasted. The rootzone is thus critically important!

How Much Water is in Soil? Consider a bermudagrass root system which reaches a depth of 15 inches in the soil. We can multiply 15 inches by 20%, giving us 3 inches of available water. This would probably be enough for 8-9 days in the summer, as will be discussed below. The bermudagrass root system would have access to around 3 inches of water, assuming the soil were at field capacity.

How Much Water is in Soil? Now consider a bentgrass fairway, with a root system 6 inches deep in May. Multiplying 6 inches by 20% gives us 1.2 inches of available water. This would be enough water for around 4 days. Finally, lets consider the same bentgrass, but during the heat of August when the root system has decayed to only the top 2 inches of soil. Two inches multiplied by 20% give 0.4 inches of water, about enough for one day.

Soil Textural Class Affects How Much Water is Held by Soil

Changes in Soil Moisture Can you guess what is happening? Soil H2O Time

Localized Dry Spots (LDS) Water won’t infiltrate, just sits on the surface. Caused by hydrophobic conditions which develop at or near the soil surface Similar to a waxy coating on the individual soil particles - Oil and Water don’t mix!

Localized Dry Spots (LDS) Usually worse in sandy soils Spotty, random distribution Wetting/drying cycles make it worse

Coated Sand Grains Repel Water H2O H2O Uncoated Sand Grain Coated Sand Grain

Hydrophobic Soils H2O H2O Hydrophobic Dry Soil

Coping with LDS Avoid allowing soil to dry out Cultivation or topdressing may help by mixing hydrophobic soil with unaffected soil Most common method is by using wetting agents

Wetting Agents Similar to soaps, but they are not designed to remove the waxy coating, only mask it so that water doesn’t know the wax is there. On small areas, can use Joy detergent mixed with water 1:1000 For big areas, numerous commercial products

Wetting Agents Can improve turf quality, root growth by maintaining adequate soil moisture May reduce total water use, which will save $$ Increased infiltration can mean less down time due to standing water

Surfactant Molecules Polar Head (attracted to H2O) Non-Polar Tail (attracted to oil, wax etc)

Wetting Agents Surfactant Molecule H2O Waxy Coating on Sand Grain

Plant Water Plants require water for one major reason and one minor reason. The vast majority of water the plant absorbs from the soil is actually lost as water vapor from the leaves, to the atmosphere, by the process of transpiration. Transpiration occurs through the leaf stomates, and is very important because it cools the leaf.

Plant Water If not for transpirational cooling, a leaf could reach a temperature of 120o F during midsummer. This temperature would easily kill most plants. Fortunately, transpiration keeps leaf temperatures much cooler, usually below 90o. A small amount of water that is absorbed is actually used to build new tissues, but for every ounce of water used to fill up new tissues, around 300-400 ounces of water are lost to the atmosphere.

Plant Water Roots absorb water from the soil and transport the water to the shoot through the xylem. But how? We understand about the force of gravity pulling water out of the macropores, but how does a root exert a force on water to pull it out of the micropores? To understand how this happens, we need to understand a few rules about water.

Plant Water First, water runs downhill. It flows from a position of high energy (the top of the hill) to a position of lower energy (the bottom of the hill). Sometimes there aren’t any hills involved, but water can still exist in high energy and low energy states. This is the case in the soil/root environment.

Plant Water Soil water at field capacity can be considered fairly high energy. The water in a root is fairly low energy. Thus, there is a natural tendency for water to flow from the soil and into the root. How does it get to the shoot?

Plant Water The second rule about water is that it is sticky. It sticks to itself, which is called cohesion. Consider what happens when you suck water through a straw, even a real long one. The water is pulled up against the force of gravity, in a continuous column.

Plant Water The force to pull the water is the vacuum, or negative pressure, created by your mouth. All the water behind the leading edge is “sticking” to the water in front of it, and being pulled along. This ability to pull long columns of water up, against gravity, is fundamental to getting water up inside a plant.

Plant Water It may be helpful to think about all the water in a plant as part of one gigantic mega-molecule. All the water is connected because water sticks to the adjacent water. Plants lose water through their stomates as a gas. This is like sucking on a straw. The water lost through the stomates exerts a pull on the mega-molecule in the plant. In other words, transpiration gives the rest of the water in the plant a little tug.

Stomata

The Stomatal Cavity

Plant Water There is one big continuum of water from the soil, through the root, up the stem, and into the leaf. When you tug at one end, the other end feels the tug. Eventually the soil water may be nearly depleted, approaching the permanent wilting point. At this point, the plant is unable to get soil water to move quickly to the root, no matter how hard it “pulls”. Water is still being lost through the leaves, but isn’t being replaced from the soil. The result is a water deficit, or wilting.

 is symbol for water potential plant = osmotic + turgor Plant Water Potential  is symbol for water potential plant = osmotic + turgor -6 bars = -8 bars + 2 bars

Water Moves from Higher to Lower Potentials -1000 bars -8 bars Air Shoot -6 bars Root Well-Watered Conditions Soil -4 bars

Soil Water Potential May Limit H2O Uptake -1000 bars -13 bars Air Shoot -12 bars Root Drought Conditions Soil -18 bars

Drought Symptoms Curling of leaves in some species Gray or blue color develops Footprinting Wilting Death

Coping with Drought Avoidance Tolerance Escape

Avoidance Usually the first response. Plants adapt to avoid internal water deficits - eg. Deeper, more extensive roots to absorb more water, closing of stomates or thicker cuticles to reduce water loss.

Tolerance When water deficit does occur in the plant, some have the ability to tolerate it. They do this by maintaining turgor pressure in the cells via osmotic adjustment. This is probably secondary in importance in turf.

Escape Annual species avoid drought altogether by surviving the period as a seed. Some warm season grasses, and KBG can survive extended periods in a dormant condition.

How Much Water Does Turf Use? It depends on the environment, the turf species, management practices, and soil moisture. Environmental factors that control water loss (evapotranspiration) are: Temperature Relative Humidity Wind speed Light Intensity (Radiation)

How Much Water Does Turf Use? Water use rates are usually expressed in inches or cm of water lost per day. In general, the warm season grasses use less water than the cool season grasses.

How Much Water Does Turf Use?

How Much Water Does Turf Use? How do we determine water use? There are a number of methods used to estimate how much water a turf requires at any given time, under any given environment. One of the most common methods was discussed above, where environmental data are used to calculate a theoretical, or reference water use. This value is referred to as ETp, or potential evapotranspiration, and it is used as a reference point.

Automated Weather Stations Both measure: -Temperature - Wind - Rel. Humidity - Light

How Much Water Does Turf Use? Another method is the Atmometer, a simple, inexpensive device that mimics a leaf canopy to estimate ET.

Atmometer

How Much Water Does Turf Use? There are also a number of soil moisture sensors on the market, including: tensiometers gypsum blocks solid-state sensors Aquaflex®

How Much Water Does Turf Use? Most of these measure soil moisture at one point, in a single, small volume of soil. This means that it might not be very representative of the overall soil moisture.

How Much Water Does Turf Use? Aquaflex® is a new product, which we are evaluating at NCSU. It consists of a 10’ long cable which is buried in the rootzone, at approximately 6”. It has the advantage that soil moisture is measured and averaged over a much larger soil volume.

Slit and Sensor

Inserting Sensor

Sensor in Valve Box

Taking a Reading

Irrigating “Between the Lines” Field Capacity Refill Level Perm. Wilt Pt.

How Much Water Does Turf Use? Actual turf water use usually isn’t quite as high as ETp. We use a factor, called the crop coefficient (Kc), to relate ETp to actual turf ET. A crop coefficient remains fairly constant for a given species during a given season, but varies considerably between species and between seasons. It is useful, however, if we have enough previous data for crop coefficients for a given species.

How Much Water Does Turf Use? For example, we know that the Kc of bermudagrass is about 0.7. This means that bermudagrass will use about 70% as much water as is predicted using environmental data to calculate ETp. If our environmental data tells us that a reference crop used 2.2 inches of water for a given week in the summer, we can multiply 2.2 by 0.7, giving us 1.54 inches of water actually used by the bermudagrass.

Water Use These calculations assume there is adequate water in the rootzone. Mild to moderate drought stress may reduce actual ET. For example, if bermudagrass is grown under continuous, moderate water stress, the turf will easily survive, yet actual ET might only be 0.55-0.6 of Etp.

How To Manage Water Based on what we’ve just discussed, we now have enough information to schedule our irrigation. We need to know how much water our particular turf is using, which we will determine using reference ET from a weather station/computer system plus a crop coefficient specific for our turf species. We also need to know where the roots are in the soil profile, at least roughly. Finally, we need to know how much available water can be held by our soil.

How To Manage Water Basically we’re going to treat our water like a bank account, where we have inputs (deposits), outputs (withdrawals), and a certain amount of water in the soil (standing balance). We just follow the flow of water (money) into and out of the system. In our case, the system is the soil in the rootzone. If the roots go down 12 inches, our system is the water held in 12 inches of soil. If the roots go down only 2 inches, our system is the water held in that 2 inches.

How To Manage Water We start with our soil at field capacity. If our 4 inch root system holds 0.9 inches of available water, that’s what we have to start. The weather station data tells us that over the first three days, 0.8 inches of water are used by the reference crop. We apply our Kc of 0.7 for bermudagrass (assuming that’s the grass being grown) and find that our turf actually used about 0.56 inches of water.

How To Manage Water Subtracting this from the original 0.9 inches of available water, we find that we have about 0.34 inches of water left. It’s time to water, since we don’t want to completely deplete all the available water. How much to irrigate? About 0.6 inches, to replace the 0.56 lost from the system, and have a little left for good measure. We have returned the soil to field capacity, without irrigating excessively and wasting water. What do we do if it rains?

Conduct a Water Audit Observe system for obvious problems Using water meter data and turf area, calculate average rate Canning the turf to determine application rate to determine distribution, uniformity to identify problem heads

Canning an Athletic Field

Canning an Athletic Field

Canning an Irregular Turf Area

Canning the Turf Run irrigation system for set time Measure depth of water in cans Average the measurements Calculate irrigation rate Average Depth/Time = Rate 0.6”/30 minutes = 0.02”/minute 0.02 in/min X 60 min/hr = 1.2 inches/hour

Irrigation Uniformity Critical to Saving Water

Ideal Uniformity

Actual Uniformity

Over Irrigation to Compensate

Center For Irrigation Technology http://cati.csufresno.edu/cit/