"Water Use Efficiency & Partial Rootzone Drying in Fruit and Nut Trees"
Organization:DNRE: Department Natural Resources and Environment
Private mailbag 1, Ferguson Road
Tatura Vic 3616
Phone: 3-58335231 0407-335 231
A scientific trial was conducted into the effects of partial root zone drying on peaches, pears and apples.
...that sort of clever irrigation. And the work initially started with Brian Lovey's from CSIRO in South Australia. He developed this technique in grapes, and since that time we have developed a national project which was funded by Land and Water, Research and something or other development corporation. So we have had a national project running for about three years. More recently, I have been working on a project funded by HRDC, looking at this technique in canning peaches. That is how it all came about.
Essentially, what the technique is, irrigating half the root zone in the orchard. In doing this work, we have three treatments, including the control. The control is where we simply irrigate the tree as per normal. In this work, we have been doing this trial on commercial orchards, so the farmer, or the orchardist, takes control of the irrigation: he irrigates in the normal fashion.
In the next treatment, one side is kept wet and the other side is kept permanently dry for the duration of the irrigation season. So we simply block off half the irrigation system. The other side is just watered normally, so effectively, we are using almost half the water.
In the third treatment, we thought we would get a bit clever, and we alternate the sides of the tree that are irrigated. We call it alternate irrigation, or sometimes flip-flop, going back to the electronics analogy. It simply means flipping from one side to the other.
These are the three basic treatments, and we set up replicator trials on commercial orchards. In the first instance, we looked at pear trees. These pear trees are about forty years old. They are simply irrigated by a pipe and riser system, as we call it. Water just flows out of these pipes, there are simple taps on the end. What we did was bypass half the system, and this pipe ran down past our trial and let the water go down to the other end. Effectively we are only irrigating one side of the tree, or one irrigation bay. This is flood irrigation. Normally the water is allowed to fill those bays and just runs downhill under gravity and flood-irrigates the orchard. This is a very simple trial to begin with. We set it up so the irrigated side of the orchard was well-irrigated. The pasture that the orchard is sown under is quite lush and actively growing. We see quite a heavy crop of pears, Packham pears, the canning pear.
If we look at the opposite side, the dry side, the grass is drying off and by the time the crop was removed, that side was completely dry. We ran this trial for two years, and then we looked at the effect of this partial root-zone drying on the tree. This is just a hypothetical situation, to show what we are doing, so our control, irrigated control, if this is our soil depth here and our range of wet and dry, this might be a typical sort of wetness profile in the soil. So the irrigated side would be wet up to where we measured the wetness of the soil, down to 110 cm, and the dry side might look like this: the roots close to the surface have extracted water and the very surface layers of the soil are getting extremely dry. The grass, the pasture, is wilting and the roots are trying to draw water from greater and greater depths and having a lot of difficulty at doing that. These roots here are under severe stress.
To try to work out what was happening in that tree, we did a number of things. One of the things we do check is the growth rate of the fruit. We simply number a series of pears and we use a set of electronic calipers to measure the fruit, or even a small tape measure wrapped around the fruit. We do this at least once a week, we plot the result, and this gives us a very sensitive indicator as to the growth rate of that fruit and what is going on in the tree, to see if the tree is under stress. It is one of the many measurements we do. It is a little harder to measure shoot growth, vigour, in a tree, particularly a mature tree. Fruit growth is often used in our trials to indicate the rate of growth in the fruit and the activity in the tree.
We also use electronic instruments to measure just how hard that tree is working, whether it is under stress. This instrument my assistant is using here is measuring stomatal conductance, or the rate of transpiration through the tree, if you like. He is clamping a little instrument on the leaf and he has a logger down here and it is measuring the rate of respiration in the tree. This gives us a sensitive measure of whether the stomates in the leaves are shutting down, or whether they are open, and the actual conductance of gases through those leaves.
We also measure leaf water potential, which is best explained, in terms of the stress, how tightly water is held in the tree. When the tree is being droughted, the leaf water potential in these leaves is very high. We do it by putting them in a pressure chamber and applying air pressure to the leaf. When we see liquid coming from leaf stem, we relate that to the pressure being used in the cylinder, and that gives us an indication of how much stress the tree is under. It's just another way of working out the tree's response to these droughting methods.
When we started looking at the mean yield and number of fruit on these trees, we found that between the control, which was irrigated normally, and the partial root zone drying treatment--this is the permanent wet and dry we are looking at--we got an almost identical yield, and yet the PRD trees were getting almost exactly half the amount of water. Every second row was irrigated. We found this rather difficult to believe in the first year. We thought, this can't be so, there has to be an error here somewhere. Perhaps the tree is picking up water from the water table. So we put down some test wells and we were monitoring soil water with a a gopher, a capacitance meter. We had tubes set out across the row, trying to work out where the water was coming from. And then we thought, perhaps there is leakage across the tree line, that the flood-irrigated side is leaking to the dry side. But that wasn't the case, because we had these access tubes across the row, so that wasn't occurring.
Now, why was the tree responding in this way? We began to ask questions. We have been looking at tree water use at the Institute for at least fifty years, and of course many other people around the world are doing it too. We thought, have we got it so wrong, or is there something else going on here. It was very, very exciting to us, because we were showing that we were getting the same yield for half the resource of water under flood-irrigated pears. We thought, well, we won't make this public just yet, we will look at it yet again. So we went through a second season.
When we looked at it, we counted the number of fruit on each tree, and we found that, yes, the PRD did have slightly more fruit. In fact, when we related it to the mean fruit mass, we found that the fruit was slightly smaller. More fruit, slightly smaller, same yield. So there was a slight effect on mean fruit size at harvest, but not enough to impact on yield. In fact, in these orchards, fruit size, oversize pears, are not all that marketable. Ideally you want an average-sized pear, so you can fit a number into a can. Often you don't want really big pears.
When we looked at tree yield across all treatments, and we have a range of treatments across here, both PRD and control. All I wanted to show was that the tree yield from each tree was up and down like that. There wasn't any consistency relating that yield. That is a normal distribution of yield per tree, because trees vary in size, some trees yield more, some less, but over all treatments there was no consistencies.
We plotted fruit size over time, from the first of December to the 14th of February, right up till harvest. We measured fruit diameter on each side of the tree, so that we could see whether there were any differences on the wet side or the dry side. If we look at the control, both sides of the tree are irrigated, we found that pear size was about the same on both sides. This is showing one of the extreme cases in the second year. We found that there was a slight penalty in size in this particular treatment. In some of the others they almost lay on top of each other. Essentially, there was no difference in pear size throughout the growing season. When we checked with the orchardist we asked if that was anything to worry about; we would pull the pin on this treatment if wished, and we can irrigate both sides and call it quits. He said, "No, no, we have been measuring pear size for a long time, we are not at all concerned about it." Yet we were still putting on half the amount of water.
So, what, indeed, was going on? Are we sort of tricking these pear trees, setting them up for drought conditions? It is intriguing to know this. We know that we are saving water. But to scientists, once you save water, that's it, end of story. OK, you have saved water, where do you go now? But the really interesting challenge was to find out what was going on in that tree, what was the physiology all about. The fact that there wasn't a great deal of difference between the wet side and the dry side suggested to us that there was transport of water across the root zone to the other side of the tree. So water was being picked up from one side of the tree and affecting the fruit size on the other. That was quite interesting.
We regard flood irrigation as a rather crude form of irrigation. It's all or nothing, you either irrigate or you don't. You don't sort of put a little bit on; you have to put everything on. It is not an acid test of this partial root zone drying system.
So we thought we would look at other crops. I already mentioned that Brian Lovey and his team are looking at grapes. Other people around the country are looking at citrus, apples, and a number of other crops. We did plan to look at annual crops like tomatoes, but we didn't quite get there.
This is a Tatura trellis system of growing peaches. These are Taylor Queens, just a standard canning peach, on the property of one of our neighbours. He is a very good orchardist. These trees are managed under microjet systems. The trees are about 2 m apart in the row and about 4 m across the row, and there is a microjet between each pair of trees. One microjet per tree, if you like. Again, we did the same thing. We told the irrigator to irrigate normally. We put in a partial root zone drying treatment, but instead of irrigating half the root zone across the row, we irrigated half the root zone down the row. With piped irrigation, it was easy to switch off every second sprinkler and just irrigate, instead of across the row, we are irrigating one side of the tree trunk and not the other. We could achieve this simply by blocking off every second microjet, and putting down a second lateral second lateral so we could switch the irrigation from one side to the other. So a bit of simple plumbing went in, and that is the way we managed it in the peach orchard.
So we see in this picture, we put in two pipes, two laterals, blocked off every second sprinkler, and with a couple of taps at the end, we could decide which side we irrigated. In this picture, you see the moss growing here: that is the irrigated side. We had a number of trees labeled in the row where we could monitor fruit growth and eventually harvest those trees and get a figure on fruit yield. There are access tubes placed in the orchard to measure soil water. We use a capacitance probe which we slip down and measure on a regular basis to work out how deep the water is going and where it is going across the row, and that sort of thing.
When we looked at Taylor Queens first, and this last season we looked at a 204 peach, which is another canning peach, very popular in our industry. We got the same result; in fact, we got an even better result. So when you look at the mean fruit growth from November through till the end of January, just before harvest, we found, in fact, that there was little difference in the fruit growth measurements, between the control that was irrigated on both sides compared to the PRD treatment where we blocked every second sprinkler. For all intents and purposes, we used half the amount of water.
It raised these questions again. How could we use this to improve productivity, improve fruit quality, the fact that we could manipulate tree water use. Growers, at first, weren't terribly excited, because the cost of water in our part of the world, through microjets, which is a pretty efficient way of irrigating, is not a major input into the orchard system. They are concerned about it, but it is not one of the major concerns. And the industry didn't get all that excited. They said, you have saved water, yes, and we appreciate that, but we are looking at productivity and we have other priorities.
But in fact, it suggested to us that we could manipulate fruit quality, because we know, with fresh market fruit, that fruit firmness, fruit colour, and lots of other issues could be affected. We have yet to show an impact on the canning peaches. In fact, we haven't analysed the data from this trial yet, because the data has just been collected. But we did get some indicators in apple crops that we did affect the firmness of the fruit--we pressure tested a lot of fruit, took dry weights, we measured sugars and acids. In Pink Lady apples there is some evidence to suggest we have improved the quality of the fruit, the keeping qualities, the firmness of the fruit, and the taste of the fruit, through applying PRD. It is early days, yet.
Here are some of the results from this last trial, we haven't analysed them all. When we look at the effect of PRD on mean peach mass, pressure, we use a penetrometer to measure the firmness of the fruit, water content and sugar content, through a Brix reading. We find again that there are very small differences between the methods. In this case it is the flip-flop, the alternating treatment and the fixed PRD and the control. The differences are not significant.
Pressure is the same. There are some very slight differences, but again, not significant. Here we got a slight drop in water contents of the fruit, and we are not sure whether that is important or not. Again, sugar content in peaches, at least, we didn't show any major differences. So, all very interesting stuff.
This is a shot showing how we measured soil water content using a gopher, which is just one of the many instruments used to monitor soil water in horticultural crops. In this photo, it shows the operator lowering a capacitance probe down a plastic tube. We can measure the water content of the soil at a series of depths. It is non-invasive, apart from the tube which you put down permanently in the orchard. We can log that onto a logger or computer. It does need calibrating for particular soils, but is quite a useful instrument to use.
When we look at the water content, and this is a sum of the water contents down to 50 cm depth, we can see some of the effects of PRD. We see that it goes through a drying period here and a wetting up period there, and then drying again, just indicating the flip-flop treatment. The permanent wet is down here staying fairly constant, and this is just indicating the dry side up there. So, just showing that the soil is wetting up and down, and other results tell us there is not a lot of leakage between the two.
This is a bit of an exaggeration of what is going on. We have had, as we saw earlier, in the alternating treatment, a slight change in fruit mass. We are talking about grams so they are fairly small differences. We are getting lighter fruit in that alternating treatment, but not enough to draw a lot of conclusions from.
Our ultimate aim is really to take this further. It is early days. We have had a couple of years of experience with flood irrigation, and we have had something like three years experience mostly with peaches but with apples, too, with microjet irrigation. Our intention is to pursue this further, to try and work out how useful this technique might be. In fact, we are quite keen to follow this through because some of the orchardists are telling us, in years where water is very limiting, say in a drought year, could the orchard be set up in such a way that you actually could bring in PRD without a yield penalty. So to use that limited amount of water you have got very, very efficiently. And, perhaps, we can use these techniques to actually impact on productivity and fruit quality, fruit colour, and those sorts of things.
Q (couldn't hear question)
A. Well, in flood irrigation, there was a slight amount of leakage, but not enough to explain what was going on there.
Q. Do you have an independent assessment of the leaching fraction of the two methods?
A. We do not. You will not that I said that we didn't make it public the first year because we had some reservations, and we still thought there may have been some channel seepage: we were quite close to a channel. We checked those sorts of things. It didn't explain what was going on there, so we reserved judgment until we got into something a bit more manageable. It didn't seem to be the case. The fact that this evidence that we had here was being backed up by other people in other industries....
Q. Just a reminder from the most efficient country in the world, Israel. My friend Isaac tells me that they had 95% water efficiency there in 1976. I talked to him again about three years ago, and lo and behold, the aquifer, recharge drainage, was far above that, and that their efficiencies were more in the order of 75% to 80%. It all comes down to being darned difficult to measure the downward flux, at that lower boundary.
A. But the fact still remains that we take away half the water supply to the orchard and still maintain yield.
Q. Is anyone looking at the relationship between LAI, ...?.depths and crop water use and crop yield? Is it such that we are very impacted here, that our yield doesn't follow the same curve. Our yield is coming up and flattening out long before our ..?..
Q. Yes. When this project started, it was only a minor project, not a lot of funding. That is one of the reasons we chose flood irrigation. It was simple to manage, and so forth. Even this latest project only got half the funding we sought. I should add, though, that other people, including Brian Lovey's and my colleague, Ian Goodwin, are looking at sub flow in roots and shoots. I forgot to mention earlier, too, one of the things we are interested in is hormonal signals sent by the roots. One of the chemical signals is abscissic acid. When the tree is under stress it sends certain signals up the transport system in the tree, and the top of the tree responds in a certain way. Sort of a stress relationship.
There are all those sorts of things. We often use the term, the 'Bonsai' effect, which tends to be a physical sort of stress set up in the tree. Again, if you restrict the root system or stress the root system, there is a corresponding response in the top of the tree. All sort of fascinating things, which we don't fully understand, but we are trying to exploit.
Q. What is your procedure for regulating the wet side when you irrigate?
A. Well, the farmer did all that. We didn't interfere with that because we thought we would introduce another factor. So our instructions were that irrigation should be done as for the rest of the orchard, and we simply took away half that water.
Q. Is there any benefit to keeping part of the root zone wet and the other part dry?
A. It is a good question, in fact it is beyond the scope of this talk. Just as bit of a teaser, what fascinates me is the idea that if we can flip-flop the irrigation, can we, in fact, use that to trick the root system, we can actually give it a spell, too. Because, if we continually keep soil wetted close to saturation, so that water is non-limiting, there is a huge flux of water there ready to be taken up by the tree, we are running very close to the edge. We run the risk of root diseases. If we get heavy rain, the soil may saturate and we introduce a problem. Perhaps, using the flip-flop technique we can build in a sort of a safety margin and perhaps drive that tree even further. It's just a theory.
Q. I was thinking more of keeping the soil below saturation.
A. I guess we are doing that already, with bad irrigation. We have decades of droughting trees. What we are trying to do is still maintain our productivity, but do clever things with water. It is a fine-tuning of irrigation.
Q. (can't hear it)
A. It raises an issue, because when I approached the fruit industry we got a sort of lacklustre response. They said they had other diseases to look at, marketing, and a lot of other reasons, so water usage efficiency was low on their priority list. But I said, with all this interest in water saving and environmental flows down rivers and creeks, it is high on the agenda of politicians. And I said to the industry that you could have high moral ground on all this. We talk about carbon credits, what about some water credits because you are doing all these wonderful things? They immediately seized on it and said they would take it.
Q. I would like to make an observation that many tree under stress produce a better fruit, they don't reduce their production of fruit. They sometimes produce more flavour and better texture.
A. Well, in the grape industry we call it the 'dying vine theory.' The worse the vine, the better the wine. You do get a concentration of sugars, taste may change.
Q. In your measurements, did you check the flavour and the texture?
A. Not the flavour. We do measure sugars, Brix reading. Again, the experiment wasn't set up to look at fruit quality, per se. We did do it as an adjunct to it. We are still looking at that. The most promising results came from apples, not from peaches.
Q. You had a bar graph there of different yields, different trees. Do the trees tend to be persistent in their yield from season to season? Would that graph look much the same in another year?
A. That was an actual yield. It was an old-style orchard, old pear orchard. Not hedgerow trees, there were large and small trees. Some of it is a reflection of the variation of tree size. There was a lot of variation in those yields. It was just an illustration of that point. One of the difficulties we have in running field trials like this, credibility is high because a lot of the orcharding community want us to work out there. The trouble is, try to find a uniform block to work with. In fact, we picked a block next door, went around and looked at all the trees. They looked wonderful in the winter, but as soon as they started to leaf out, we found that some barely leafed up, they had root disease, or whatever. So we had to shift our whole trial. We found it really difficult to find a 100 trees of about the same size. It is just one of those things when you are running trials, to start with a uniform population. It can be very difficult.
Q. In your measurements of fruit size, did you notice any variation with relation to natural rainfall?
A. The rainfall tends to be pretty low in summer. There would be a slight effect, but the biggest impact would be from irrigation. We do use fruit size measurements in other trial as an indicator of soil water, and rain would be part of that, topping up.
Q. Where did you access your water from?
A. We had reservoirs and distribution channels throughout the countryside, and so on.
Q. Is it really half the water? Were you measuring the runon water?
A. No, we were not. In our press releases I think we took the conservative line, we said, "Up to 40%." We can't quantify it, really. We did measure the water on--we have water meters, but explaining where that water is going is just beyond the experiment. We have been challenged on oll these matters. Still, it is such a major shift in our thinking. We haven't demonstrated, say, a 10% saving in water, which you could explain away in various ways, particularly with flood irrigation where you have a whole bucket of water going in on one side and not on the other. It is a challenge.
Q. A comment, addressed to Harold: the whole essence of your talk was all these people who have more water than they need. In WA, the people down south who want to grow fruit can only get so much from the Harvey dam. If they could grow double the fruit....
A. Yes, it is an important issue. When I think back, in the 50s, flood irrigation was the go. We had some furrow irrigation in orchards, then we put in those pipe and riser systems. It was a major leap in technology to have water on tap. But we are still flood irrigating. And then, about 1970, we adopted drip irrigation, and we played around with it, we got it all wrong, and a lot of people didn't believe that drip irrigation could supply the needs of a large tree, that sort of thing. We made a lot of mistakes, we killed a lot of trees. A whole lot of research went into tree water use.
Before drip irrigation, we had monsoon type sprinklers--the brass, knocker type sprinklers. We put those through the orchard. One of my first jobs in the 70s was to measure sprinkler distribution. We put out rain gauges and we discovered that the sprinklers put out these doughnut wetting patterns. In fact, when we looked at an aerial view of the orchard, the outer limits of that wetting pattern, you could see that all those trees in this particular block contracted phytophthora. As the stream of water struck the tree trunk, it created this massive puddle near the base of the tree, and root rot set in.
Then we went to microjet, and that offered us slow wetting, reasonable distribution if you have the right microjet, and so on. So all these are a step forward in improving water use efficiency.
Q. Does flood irrigation use twice as much water as microjet irrigation?
A. Certainly, there can be wastage. Flood means a lot of things. I shouldn't condemn flood too much. I am thinking of it from a soil scientist's point of view, and I know some of the damage it did. In fact, with flood, because water is, in theory, at least, moving downwards, the problem is to make it uniform and keep it under control. Without some form of recycling it is very difficult to make good use of it. You need to weigh that up with how much the crop is actually picking up. Irrigating pasture, the entire cropland is covered with crop. So there are a few factors in there, but essentially, we are getting better at it. The other thing is that we are capturing a lot of this water before it enters the water table. In the worst scenario, a lot of the water moves past the crop root zone, is not picked up and gets into the water table, and problems arise. If you can just metre on enough water so that it is picked up by the crop, and so on, there are some efficiencies there.
Q. In the evolution of irrigation methods, what is the latest for things like nut crops?
A. I can only speak for our region and our soils, but almost no one puts in drip now except in vineyards. Vineyards and drip go together, closer spacing and other factors. Our soils slake and disperse pretty readily, they are shallow. Infiltration is low, and what we need to do is wet that soil slowly and evenly. It is a bit hard to do with a drip, because it is like a mini-flood irrigation under the dripper. Getting uniformity in these soils, clay-loams, largely, is fairly difficult when water is coming from a point source. When you are spraying it in the air, it is easier to spread water in air than it is in soil. You can get it on reasonably evenly in that wetting pattern from the microjets. Ideally, if you could wet it slowly like blotting paper, in theory, so it wets by capillarity and doesn't completely fill the soil with water, so it doesn't saturate, that would be even better. There is less risk of soil damage.
So all those factors come into play. It is not simply just plumbing water to the trees. More than clever plumbing, getting water to where the roots are is the soil between the emitter and the tree roots that needs to be considered.
What we do is arrange our microjets so the water jet doesn't reach the tree trunk; it stops short of the trunk. The only thing that changes that that is acceptable, not desirable but acceptable, is with a very young seedling tree, or a young tree in the first year, because the root system is very limited. But then we move it away so the patterns just stop short of the tree trunk.
It's interesting, we get into this debate with new developments, particularly in the mallee region in Victoria. Some growers like drip because it is cheaper, because the hydraulics are simpler, so you can use smaller pipes and pumps. The tendency is to use two lines of drips so you have a sort of backup system... (tape ends and is turned over)
....from a lack of maintenance point of view. We have this continuing debate that has been raging for several years.
Q. Yesterday a fellow named Andy Wright talked about sandalwood up on the Ord River and the amount of water they pour on that country. It has taken us a hundred years to fill the land of the wheat belt up with water, I'm just wondering if we are going to see that water up in the north come back to bite us on the back side? They just pour it on with no regard to the amount. Are we doing the wrong thing? Should we be conservative and only put on what the crop needs and let the rest flow down the overflow?
A. Again, you need some form of control in the water. With the microjet you have much finer control with how much you can put on. You can, in fact, just put on a millimetre of water if you want, in theory. That is a great asset, to be able to do that. With monitoring, like that gopher I showed, or potentiometers, you can wet up that soil so that it is at the optimum for tree roots, but minimize the losses, what is draining through to the water table. But you really can't do that unless you have the control, your hand on the throttle, and something to tell you how far to go. So you need the two: good, well-designed system and you need a measure of how wet the soil is. Otherwise you are flying blind.
(Comment from the audience:) The water table in the Ord district has risen by 16 metres since it started.
Leaving that Tatura back in the mid 70s, we had ground water, and these are clay soils, within half a metre or less of the surface, we were losing orchard trees everywhere. The way the problem was addressed was to put in groundwater pumps and to lower that water table with strategic pumping. Jim knows about this. Those pumps still protect the orcharding areas. It is not appropriate for all areas, it is expensive because you need pumps throughout the district. In fact, I installed the very first pump at the Institute. It does work, but then you have to decide what you do with the water, where do you put it. It is something that you never get away from. You need to manage that water forever.
Q. By putting on half the water, you didn't show an increase in sugar content, you showed a decrease. To me that says the controls were putting on way too much water, and even if you put on half as much, it is still too much water.
A. I tried to show, this is our first look at this. Because it is such a drastic measure, and we haven't yet got into the fruit quality because our project was a fairly small one. We did show it in apples, but I didn't present those results today. But it does suggest that you are exactly right. We haven't got it right and there must be room in there to maneuver. That is really the challenge I am throwing out today.
The situation of putting on too much water represents what the industry has been doing for decades. That is the standard we have operated at. It does suggest we can raise the bar a bit. We can say, Let's target this a bit better. It is a fascinating thing to look at. It was on Landline a while back, but Tatura didn't get a mention. We are rather peeved about that. CSIRO being what they are, they don't mention Department of Agriculture. We are good friends, though.
Q. The Men of the Trees have a trial plot at Amory Acres just east of Dowerin. We are on a duplex soil, sand over clay, a non-wetting topsoil. This gives us enormous problems in irrigation. We found the best way of going about this is what we call sub-watering. We put a pipe alongside each tree, which is 65 to 90 mm, and it goes down probably about 40 cm in the ground. We just dump the water into the pipe to get it underground as quickly as possible. If we hit the surface, it just seals off and evaporates, as well as encouraging weeds. I just wondered if you have any experience of this method.
A. In fact, it is a little like buried drip, I suppose, where you bury the drip below the surface. I have some concerns about that because you are putting a big slug of water into the soil, and it is wetting the soil quickly, and in some soils that leads to slaking and collapsing of the soil structure. If it is a very, very sandy soil where structure collapse is not a great concern, it could be done. But it depends what your priorities are. If water is seriously limiting and it is the only way you can make it work, fine. But if you had a choice, we would much rather see the soil wet slowly and carefully, rather then putting a big slug in.