Hello, everyone and welcome back. Today, I want to complete our discussion

on irrigation. If you recall, we left off talking about

irrigation systems and a little bit about soil water holding capacity, and some

methods to tell when to initiate irrigation.

And today, I want to continue on talking a little bit more about ways to determine

the amounts of water to apply. We want to focus a little bit on a couple

systems. One of them sort of takes off on the, the

use of the soil moisture indicator devices.

And then, the other one is the method that we call the water balance, or some call

the checkbook method. So recall, when we talked about

tensiometers. We said that they were, they were good

tools to give us an indication of the soil moisture, and we talked about how they

worked in having a tensiometer tool that's with a column of water that's continuous

with the moisture in the soil and in as the soil dries out, it draws on that

continuous column of moisture and registers a pull or a vacuum on the gauge

that we can measure. And we can relate that number to how dry

the soil is and if we have that system calibrated well enough, we can actually

tell when to initiate an irrigation. So, for example, I've given you some

numbers that might apply to sandy soils. Some numbers that we use here in Florida

for using tensiometers. So, for example, if the tensiometer on our

sandy soil at, say, 8 inches deep in the root zone for vegetables, when it reads

minus 8 to minus 15 centibars then that's sort of in that range, is where we want

the soil moisture to be. Now, notice I said minus 8 and that's

because this is a tension. And so technically, minus 8 is the correct

way to express the number. But by convention, most, most people that

use this system leave the leave the negative sign off and we just talk about 8

or 15 centibars. If the soil is a little heavier, for

example, a, a loamy soil, that irrigation or that ideal soil moisture regime might

be a little bit higher. So, if you have a tensiometer in the

ground and you're watching it over the course of the, the growing season and

you're watching the gauge then if the gauge approaches 8 or 6, the lower number

that indicates wetter soil. Remember the, the tension is what we're,

we're reading. And so as the tension increases and the

soil dries out and we approach a number of, say, 15 we would, for sure, want to be

initiating an irrigation event as that number, as that gauge approaches 15.

So, using a tensiometer to initiate an irrigation, or a TDR, or some other soil

moisture device often times takes a little bit of trial and error and hopefully you

have some research with these tools in your area so that you can then have some

numbers, some guidelines to go on. And so, it's important to have some local

recommendations as you consider using something like a tensiometer or a TDR in

your area for scheduling irrigation events.

Now, the reading with the, the tensiometer or the TDR or some other sort of moisture

indicator can give you a good indication of when to irrigate.

You still need to think about how long to run the irrigation system, how much water

to put on the crop because these soil moisture devices are, really seem part of

the picture and that's the soil, that's the soil moisture and we're not yet taking

a look at what amount of water the crop is actually using.

And so, what I would like to talk about now is to move over a little bit and, and

take a look at the other aspect the crop water use aspect.

Remember we've talked about ET or evapotranspiration and what that is.

Well, now, we are going to start using that knowledge in terms of figuring out

how much water to put on our crops. So, to do this, we're going to talk about

soil texture and we're going to talk about water holding capacity two things that

we've learned previously in the course. We're also going to be interested in the

rooting zone of our crop. Because that is the soil volume that we're

interested in knowing about the, the moisture content because remember

available water holding capacity in our soil.

And crops, different crops can have different rooting zones depending on the

depth of, of the roots in the crop. Some can be as far down as 4 feet deep, at

certain points during the, during the growing season.

We're also going to be with this system, we're also going to be measuring

evapotranspiration. We know what it is, but how do we measure

it? And we're going to learn about that.

Then, we're going to put all of this together, and we're going to visualize our

root zone and the water holding capacity in that root zone and we're going to think

of this in terms of a reservoir of water that the crop has to draw.

And knowing how much water the crop is using by our ET estimates, we can then

calculate and estimate how much water we need to apply to replace the water that

was taken out of our reservior by the ET. So how do we do this?

To get started we need to start at a point that the scientists, the biological

engineers call reference ET, and this is simply the water that's lost from a fairly

well level grass crop. Grasses are usually used as the, the

starting point. Sometimes alfala is mentioned in the

literature as a good reference crop. So, this is Reference ET.

This is the amount of water that's lost from a well-manicured, uniform grass

cover. So, here are some well, let me back up a

second. There are a couple of ways to estimate

this number. You can use some sophisticated equipment

to measure evap evaporation from that surface, that surrounding area.

But more likely, the biological engineers and the scientists that work with this

system had more depended on measuring certain parameters, certain climatic

factors and physical factors associated with that well turf well-manicured turf

area, and they feed that information into some models for example, the

Penman-Monteith equation. And they can calculate ET, reference ET.

And this is becoming more of an accepted way around the world to, to estimate a

reference ET. And this Penman-Monteith is a very common

way of, of doing it. Here are some numbers.

I just picked some numbers for Northern Florida around in this area the campus

area, and you can see they're expressed in inches of water lost, or ET per day.

And so, this is, if you can imagine, just visualize an inch-depth of water being

lost from that grassed surface, that would be an inch ET.

You can see though that we're talking about fractions of an inch.

And so, in January and February which in this part of the, you know, the world here

in Northern Florida is very cool. Short days relatively cool temperatures.

And then as the, the season progresses, the temperature gets warmer, the day gets

longer, the sunlight, the radiation, solar radiation gets more intense and the

evapotranspiration numbers increase. And that makes, you know, that's pretty

logical, as it warms up and you get more sun, longer days and so forth, the

evapotranspiration, the loss of water from the, from the grass should be more.

And as you go into the summer and, of course, here in this part of Florida in

the summer time is very, very hot and humid.

And so, you can see you can get up almost to 2 10th of an inch of water loss from,

from the grass surface. Notice also though, it's kind of

interesting that as you get into the later part of the summer, August and September,

you see the numbers actually go down. Well, August and September are some of our

hottest months. But also, the way the, the, the equation

works it factors in the climatic conditions of that time.

And that time during the year, we get a lot more cloud cover, the relative

humidity is higher, and those factors combined to actually reduce the

evapotranspiration. And that might seem fairly logical to us

also. And then later in the season, as the days

get shorter and the temperatures are cooler and less solar radiation, the

numbers go down and you can see, there's a very small amount of water lost from the

turf reference turf during that month. And so, you can convert these values to

gallons per acre per day, recalling that there's 27,150 gallons, plus or minus of

water in an acre inch of water. So, those numbers can give you the will

give you the, the information you need to convert the ET in inches to gallons per

acre, per day. So, for these ET, reference ET values, we

need to use values that are developed in your area.

For example, you would not want to use reference ET values developed very far

away, particularly if it's in a drier climate than yours.

Fortunately it's getting more and more common now to have these kinds of

reference ET numbers available through the Internet.

And so, some farmers have access to these ET values on a day-to-day basis.

If you do not have that available to you, there are published values, tables of

reference ET for various areas. And, at least they give you a rough idea.

You might have to use averages monthly averages or something like that if you do

not have access to real time data. For example, The University of Florida has

the, what we call FAWN, the Florida Automated Weather Network, and if you go

on this website you can see, on a day-to-day basis the ET values for several

actually several dozen weather stations scattered around the state.

So, so, it's very likely that a farmer would be in fairly close proximity to one

of these weather stations and be able to get fairly accurate and timely reference

ET values. So now, that's the ET, that's the

evapotranspiration of a well-manicured turf grass.

But we're not growing turf, we're growing other kinds of crops, cotton, corn,

peanuts vegetables for example. And so, how do we adjust that reference ET

value to a evapotranspiration value that's more close to our particular cropping or

production system in the field. And that's done by applying a, a

coefficient called K. And these coefficients adjust the

reference ET based on, mostly on the stage of, of crop growth and it's mostly re,

related to how well-developed the crop is and how much of the ground is covered by

that crop. These coefficient values would have to

come, you know, for your crops and, and for your production area.

I've given you an example of how a set of K, K values, or coefficient values might

look. I didn't identify any particular crop but

just realize that the coefficients are slightly lower early in the season and

then they grow with the, the development of the crop.

And for some crops that have a really, really high water use the numbers can

actually go slightly over 1 1.1, 1.2 at certain parts of the of the year, of the

growing cycle. So, depending on where we are in the, in

the growth cycle we would use the, the coefficient to adjust our reference ET.

So, let's look at an example for a sprinkler-irrigated crop, some crop on

sandy soil. Now, let's say that we looked up on the

Internet or on the website or we have access to some good reference ET data, and

we found that at this particular time during the growing season, the reference

ET was 0.15 inches. And we can do the calculation and see how

many gallons that is per acre per day that's lost from that crop.

And then, we also know that where the plants are growing, they're actively in

the active growing stage, and our coefficient, our published tabulated

coefficient for that time of the growing season might be 0.8.

So, we multiply those two together and get 3,200, 3,200 gallons per acre of crop

evapotranspiration. Now, we've converted it now to the crop

basis. Now, throw a little curve ball in here for

you. Remember, we talked about irrigation

systems, and the fact that they're not all a 100% efficient.

So, if we know how much water our crop is now using, and we want to replace that

amount of water, which is really the objective of irrigation remember, to

replace ET. But we know that our irrigation system is

not a 100%, then we need to adjust our crop water ET to account for that

relatively small efficiency. So, our 3,200 now becomes 4,270 gallons.

Because we may, we recognize that with a sprinkler irrigation system, we may lose

some of that water that we're pumping to evaporation in the air.

There might be some leaks. And so, if we know about the irrigation

system, then we can adjust. And, and even the book values of 75 may

get adjusted depending on your knowledge about your irrigation system even though

it says, sprinkler, if you're using drop nozzles, remember those are more efficient

than impact sprinklers on the center pivot.

So now, we know, we have another number we call our irrigation requirement.

It's a little bit greater than our actual crop water need, but we need to supply a

little extra to take, to take account for that small amount of water that's going to

be lost in the system. Now, sometimes the use of this approach

doesn't necessarily have to be done on a day-to-day basis.

You can, you know, you get into a week where the weather is fairly uniform.

You could do this on a, you could adjust the, the, the irrigation based on maybe a

two or three-day period. And so, I just wanted to make that point

that, you know, the weather is fairly uniform, you may use values ET, you know,

reference ET values, that might be the average of a two or three-day period or a

week a seven-day period. Now, we also have to realize that there is

the soil aspect to this. We now have calculated out how much water

we need to apply to that crop to replace that ET value for that crop.

But what if it rained yesterday, or last night, or the day before?

We need to, now, we've taken a look at the crop side of things, but now we need to

consider the soil and how much water is in the soil so that we can better gauge

whether or not to irrigate during a particular time.

So, let's just kind of step back and, and take a look at the soil.

Again, let's assume we're working with sandy soils.

And our sandy soil has a 0.75 inches of available water per foot of soil.

So remember, the last lecture we talked about available water holding capacity and

depletion values. So, we're going to, we're going to need to

recall that information. And let's also say that our root zone is 1

and a half feet deep. So, our crop is fairly well-developed, and

the roots are 1 and a half feet deep. So now, our root zone and the available

water that's in that root zone now, is 1.12.

And you just multiply 0.75 by 1.5, and you get 1.12.

So, that's how many inches of water we have available in that soil.

And again, we can convert that to a volume.

And let's further assume that we're going to, we're [laugh] we're little bit risk

averse. Okay.

So, recall back about allowable depletion. And so, 30%, we're going to allow, we're

going to allow our reservoir, our 1 and a half foot deep reservoir of water that's

available to the crop. We're going to allow that to deplete 30%

and at that point, we recognize we want to be irrigating.

Now, there is still water left in that reservoir, but on sandy soils, sometimes

farmers like to, to play it a little bit on the safe side and not let it go down

too much. Surely, 50, 50% might be acceptable but

recall, I painted 60% yellow on the allowable depletion.

And so, somewhere in there, depending on your you know, comfort zone as it will,

you're going to, you're going to determine some kind of a, a depletion.

And so, if we take 30% and we calculate that out then, now our reservoir that

we're managing is going to be about 9,000 gallons.

And on heavier soils, we might choose to have a little bigger reservoir and it

might be, you know, maybe 50% like 15,000 gallons.

So, this is the amount of water we're going to allow the crop to, to use and

withdraw and as soon as we see about 9,000 gallons disappear, we're going to

replenish that reservoir and add it back. So now, recall our irrigation requirement

is 4,270 gallons per acre per day. And we're going to, we're going to

irrigate when that's depleted. So, if you follow the calculations here,

if we do not get any rain it's going to take this crop at this level of ET about

two days to exhaust that allowable depletion, and it'll be time to initiate

another irrigation. The irrigation application rate of the, of

the system that you're using needs to be factored into this.

Now that we know how much water we need to put on we can determine how long to run

the system how long it takes for the pivot to go around, for using the lateral move,

hand-move system, you know, how long to allow it to stay in one position before,

before we move it. So, with this kind of information, we can

then determine the length of the irrigation event.

Now, just a few other points to make about this whole method here.

As the crop grows, the root zone is going to change, so early in the season, later,

later, and eventually it reaches sort of a, the full capacity.

So, that means our reservoir is going to change a bit during the season so we have

to account for that. Also, if we get a rainfall somewhere along

the line, we've got to add that in to our to our equations so that if that rain

added two days worth of ET, then, the that two-day period has now been satisfied

again and we start all over letting the crop use the, use the water down.

And so, rainfall has to be factored into this.

Also, this, this whole system relies on the fact that we're not farming in a in

field with a soil with a hard pan, or somewhere where there's a water table that

can contribute to the, the irrigation or the water needs of the crop.

So, if that, if that is the case then we've gotta factor in how much water is

being supplied to the crop from that from that water table.

And also note that the, the effect that increasing irrigation efficiency has on

this whole system. So, as farmers in, increase the efficiency

of their irrigation system by changing some of the technology and switching out

certain nozzle packages for example as the increase, as the efficiency increases then

that reduces the irrigation requirement. So, I hope you see how that math works

out. So, if you're in a situation where you can

make some changes, and for example, if you can get some incentive support to make

those changes, it's well-worth doing it because you'll save water.

And this is called the, the checkbook approach.

So, it adds in the irrigations and the rainfalls, and it subtracts out the amount

of water that, that the crop uses and you can add and subtract as you go along.

In fact there are websites out there and I've given you some references to some of

these on, on our course website that will help you get started with this particular

water balance or checkbook system to show you how, how to setup a spreadsheet and

how to keep track of it in, in real time. Now recall, we, we chose a, a fairly

comfortable 30% allowable depletion. Some growers I know, on sandy soils are

very reticent to go beyond that, in fact, you know, in other words, they like to

irrigate very frequently. So, some growers are inclined to keep our

reservoir filled and, and topped off with very frequent irrigation.

And I guess there are a couple reasons to caution against this general approach to

irrigation management. For one thing keeping the soil moist maybe

all the way up to the surface through that kind of an approach, encourages fairly

shallow root zones. And so, lodging to wind and things like,

problems like that might be enhanced with that kind of a irrigation strategy.

Also when you keep the reservoir filled or very, allow it only to deplete very

slightly, as the reservoir is filled, that means there is no, there is no free board

to, to, to hold rainfall. So, when a heavy rain comes, and your

reservoir is always is, is already filled, then the rainwater has to go somewhere and

it's going to push the water down. And much of the water that was in the

reservoir is going to be pushed below the root zone of the crop.

And if you have fertilizer, particularly mobile nutrients like nitrogen in the root

zone, in your reservoir, then those nutrients are going to be pushed below the

root system. So, those are two reasons for being a

little more careful about managing the water and leaving some free board in that

reservoir. So, for sprinkler irrigated for example,

for a pivot, from what we've talked about here, we need to know something about the

application rate of the system. For a pivot obviously, we can adjust the

travel speed to achieve the desired irrigation rate.

If you have heavier soils you're, you're going to be dealing with the potential for

a larger reservoir so you may be able to go more days in-between an irrigation

event, particularly if you choose a 50 or a 60% allowable depletion.

The rate of the irrigation is important. Remember, we talked about hydraulic

conductivity of soils, and so we want to make sure that our rate of application of

water is not going to exceed the hydraulic conductivity of the soil and end up with

flooding or, even worse yet runoff from the field.

And also, just another comment about the crop stages you know, some crops, like

some vegetable crops are very sensitive to even small or short timed drought stress.

And so, we need to be particularly careful during those stages of the production

cycle that we're, we're practicing good irrigation management and keeping enough

water available in the soil. Okay, here's another example.

This one is a little bit different. This is drip irrigation.

This one is different from center pivot or sprinkler irrigation because with drip

irrigation, we're just irrigating and managing a small volume of soil under the,

the plastic mulched beds, for example. I've given you a reference here that is

probably one of the, the only ones that I can find out there, it happens to be from

Florida dealing with drip irrigated crops and calculating amounts of irrigation

based on the water balance approach. So, let me just kind of take you through

this one as well by giving you sort of my rendition, my artist's rendition of a bed.

So, here's the soil, the brown, and this bed is going to be covered by a plastic

mulch. We are growing tomatoes we have drip

irrigation. Here is the drip tube.

Remember what drip irrigation is. This plastic tube then extends down the

row and wets a small area. In this particular case I've chosen an

area of soil under the, in the, in the bed of one, one foot by, by one foot.

It's actually going to spread out a little bit more in the bed than, than I, I was

able to show here in this picture, particular diagram.

So, let's assume we're growing tomatoes on 6 foot centers and, remember back to our

linear bed foot discussion, so here's another really practical application of

linear bed foot. And so, you should know now to take 43,560

square feet divided by 60 and you get 7,260 linear bed feet.

So, that's how many feet of drip irrigation we're going to have in this

field. And again, let's assume that we're working

on a sandy soil with 0.75 inch per foot water holding capacity.