Morozov A.N.

INFLUENCE OF IRRIGATION TECHNIQUE ON RECLAMATION STATE OF LANDS

We shall try to clarify a question - how and upon what do various loss items (water consumption) influence when irrigating with various irrigation techniques.

A) Deep discharge (DD) - by this term usually they interpret water penetration lower design soil root-inhabited layer. Whether it is injurious or good, if it includes (or must it include?) so called "leaching portion", number of answers to these questions is as many as researchers' number is.
We will consider that as a water volume in excess of design rate including leaching component.
Under different irrigation technique DD appreciably differs qualitatively both by spatial spreading over field area, and by desalination efficiency of root-inhabited layer. So, DD when sprinkling is a hard-hitting effect and it can be realized only if specially. Under furrow irrigation DD happens at the first half of furrows owing to necessary water running towards the end part for moistening up to adjusted rate, and it too adversely affects general reclamation state of the territory. At that, local pressure on down-situated territory of the field is formed and neighboring fields because of so called "spreading knoll of groundwater" (thanks to transfer of hydrostatic pressure). This effect is difficult to picture on flatness, and one has to be capable to apply three-dimensional graphics. When irrigating by flooding and filling along lines this one becomes apparent most brightly and its consequences are very hard to eliminate.

B) Surface discharge (SD) is imprescriptible from all ways of surface irrigation, and its role in irrigation erosion and worsening lands' reclamation state due to silting exporting drain lines and collectors is difficult to overestimate. In accordance with data of some authors, wash-away of fruitful layer when irrigating may reach very large extents (estimated according to fertility loss). SD can occur also under non-correct selected rain intensity or sprinkling irrigation technology.

C) Very disputable expense item, on which many dilettantes of science and engineering reckon, is water consumption to evaporation when irrigating by sprinkling. Let's attempt to consider it comparing with evaporation when implementing irrigation by other ways in order to separate in fact water consumption exceeding other those when undertaking irrigation by other ways and evaluate them from standpoint of influence upon soil reclamation state. For a start, we're going to remind water is evaporated while absorbing warmth of 2.45 Mega-Joule/kg. This means under water availability, factor that limiting evaporation is warmth. Under the conditions of Central Asia's south (latitude 40 North) coming solar radiation is able, in accordance with FAO's data, in the hottest year's month - June to evaporate on the average 17.3 mm/day, or 173 m3/ha/day. If to take into account that under such climatic conditions during irrigation days and first days after irrigation, water consumption by a field irrigated by surface ways reaches an evaporation value (i.e. similar value - 150-170 m3/ha/day), difference falling at evaporation part in air during irrigation makes up a trifling value ridding somewhat modified geometry of evaporating surface while sprinkling. (One can get to know results of unusual investigations on this problem in more details in the monograph of Vitaliy Konstantinovich Sevryughin). Let us examine this effect from the standpoint of influence upon the territory's reclamation state. At first, sprinkling harshly reduce depression of plants by hot weather; at second, water evaporating in air takes away warmth necessary for soil moisture evaporation, in other words, as if screens the one from evaporation. In this respect, problem of so called "efficient" and "non-efficient" precipitation is very interesting. I think precipitation of any size is efficient, even if that is completely evaporated, not getting into soil, as it keeps from evaporation quantity of soil moisture equal to itself.

But let's suppose both the authors and FAO are seriously mistaken and, indeed, evaporation in air when irrigating amounts to 30 % and more, as haters of sprinkling irrigation way assert. Even in this case, it is better to evaporate this water than to lose it at DD (which reaches 50 % and more on highly water permeable soils) and then to rack one's brains: how to fight against soil swamping and salinization, and where to how to get off the brines!

Great influence upon reclamation state of poorly drained territories is made by overflow of water, having formed by DD under flat relief, from irrigated and leached fields to neighboring ones. And the higher is coefficient of land use and poorer is degree of drainage the more serious is the problem, since in this case because of transfer of hydrostatic pressure, alternate water delivery to any field causes rise of mineralized groundwater on neighboring ones. That is, one can characterize the situation as "to mill the wind". Dangerous influence of this effect upon lands reclamation state was noted by Averyanov S.F., Shahumjan V.A., Fedorov B.V., Mihkelson B.N.

Figure 1. The Influence of the relief and zones of the feeding ground water on
nearby territory.

Even more serious consequences are caused by water DD when irrigating on a locality having big surface gradients and billowy relief as well. At this point, such cases both increasing zones of groundwater pinch existing long before irrigation and rise of new ones, especially during irrigating high parts of the relief. Such effects become brightly apparent on tapers of river carry-over, on slightly sloping and slanting submountain plains and during irrigation of so called adyry (ostantsoviy heights) and hilly territories. Application of perfect irrigation facilities on such lands could solve reclamation problem on the whole on contiguous, lower-situated territories.

Application of perfect irrigation means on such lands could solve a few problems simultaneously:
- saving of scarce irrigation water and reduction costs to generally unnecessary machine lifting water up to these lands;
- rise of crop yield;
- reduction by several times of needs for drainage on adjacent, lower areas by several times;
- improvement of the ecological state of groundwater;

Figure 2. Formation scheme of zones of groundwater pinching-out on cross-country relief of the district and, in particular, on adyrys.
By reduction of water losses at the upper part down to the minimum, one can considerably reduce their pinching out at the lower one.
One should note the fact if using perfect irrigation technique on adyrys or, generally, under automorpic conditions solve the problems mentioned above, so under the conditions of semi-automorphic and hydromorphic soils, local application of the perfect irrigation methods will be much less efficient from the water saving standpoint. (Figure 3). Then, one will have to supply water amount not less than on the neighboring areas, where the perfect irrigation technique is not used; otherwise, salinity will be inevitable thanks to subsoil overflow towards these local sites, which will play the role of "dry drainage".

Figure 3. Scheme showing inefficiency of local using perfect irrigation technique.

As to salt regime of the soils and doubts regarding to that if one ought to use perfect irrigation technique on soils subject to salinization, we will say with confidence that any irrigation ways those improve water distribution uniformity over a field are welcomed. More details of this you can find on the page "Watering technique is a key to reclamation of salted lands".

22 февраля 2004 года.

 

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