PROBLEMS AND OPPORTUNITIES OF LOW-PRESSURE DRIP IRRIGATION WITHOUT PUMPS
Dr. Eng. Dimitar K. Georgiev[1]
Drip irrigation is a comparatively new irrigation method applied wider lately due to water shortage, intensive and competitive farming. The basic advantages of the said irrigation method are manifested in rather high yields, water savings, high coefficient of its utilization and high output. In the same time should be considered also the comparatively high energy consumption compared to the most frequently applied and traditional water application method - surface (gravity) irrigation where power consumption or the size of water head respectively is insignificant.
An issue arises - is it possible the dripping irrigation method to be brought closer by energy consumption to gravity irrigation where pumps except lifting ones are practically not used. It should be noted that treatment of that problem involves preliminary disregarding the part of energy in dripping irrigation corresponding to reduction of water application rate resulted by reduction of the physical evaporation from the soil. In pursuit of this purpose and on the other hand bearing in mind the standpoint beyond doubt that an engineering decision aimed at certain effect is so much rational and technically perfect as the energy consumption is smaller then the main objective of this report will be to reveal the problems and possibilities for gaining the planned objective without altering the basic principles of dripping irrigation. Closer scrutiny of the dripping installation shall reveal that the dripper as immediate supplier of water to the soil is the main cause to use comparatively high operating head (energy) in irrigation cells. At present series of manufacturers all over the world are offering drippers with operating pressure ranging most often from 8 to 12 m with average dripping rate of 4 l/hr. Or expressed in technical terms this will mean that these irrigation drops could be obtained by raising the piezometric water level in the irrigation hoses upstream the inlet of each nozzle to height 8 - 12 meters and that pressure should reach zero value at the end of the nozzle. The basic objective of that scheme is to draw the hydraulic parameter “flow rate (q) - pressure (H)” of the nozzles as near as possible to the axis of pressure H in mode that is close to the turbulent one and than to gain greater length of irrigation laterals (hoses). The experimental analyses carried out by the author as well as hydraulic analyses are showing that quite low pressure, close to the zero, shall be required to create flow rate in the form of drops, observing some drop-forming rules.
Bearing in mind the aforesaid and knowing well the hydraulics and the other possibilities to gain uniform distribution of flow rate the author has developed on modular principle, constructed and tested low-pressure drippers with long run of water and variable length of drop-forming canal along the laterals. The operating pressure ranges from 0.5 to 1.5 m eliminating thus in most of the cases the need of pumps. The desired uniformity of flow rate distribution is gained and at thee same time possibility is created for comparatively much larger cross section of the operating canal and the danger of clogging is reduced. Furthermore, it becomes possible to reduce the requirements to irrigation water quality and consequently to change the technology and technical decisions for its treatment. The developed dripper structure is installed parallel on the pipeline (Figure 1) and presents a labyrinth with 4-mm2 cross section of the canal. Figures 1B and 1C show a provisional application of dripper in an installation with pulsating drip irrigation using a hydro-automated tank with multi-positional switching over.
Fig. 1: An installation with pulsating low-pressure dripping irrigation
a) long-way dripper with variable length of operating canal; b) scheme of the installation;
c) hydro-automated tank with multi-positional switching over
The relative energy consumption of used nozzles could be considered as main parameter and basis of comparison for energy consumption of a certain dripping installation. Figure 2 presents hydraulic parameters of drippers manufactured by various companies and a ‘section’ of the parameters regarding energy consumption at a comparatively frequently used flow rate - 2.5 l/h. The relative energy consumed by one dripper for the various nozzles is calculated with the same volume of water - 1 m3, with the time of operation and the flow rates of nozzles being reciprocal values. The table submitted below shows that energy values obtained in that way are varying within rather wide range of energy consumption by drippers. Lowest is the consumption of the TUFFIF GREEN - 8 nozzle of the PLASTO Israeli Company and highest consumption is of the DLKO2-1 nozzle of the TORO Ag (USA) Company. The consumption of the second nozzle is seven times as higher as that of the first one. In the same time should be noted that the lateral potential length of the most energy consuming nozzle is quite great - depending on the ground slope it may reach 200 - 300 m but such a length practically is not needed in most cases.
Excluding the varying prices of these installations per decare offered by the different companies the submitted results are indicative and are to be taken into consideration by future and present landlords and farmers. In addition should be born in mind that such systems work for a comparatively long period of time - 15 to 20 years, and sometimes they operate throughout the year (when two or more crops are cultivated). In regard to the potential comparatively great length of laterals in more energy consuming drippers should be noted that for the most part it is not used in and the actual length us rather smaller and dictated by some agronomic and farm management considerations - roads, canals, boundaries, size of crop rotation plots, etc.
Energy consumption in KWh x 10-3 by one dripper for
1m3 delivered water with flow rate 2,5 l/h
№ / Dripper / Energy consumption / № / Dripper / Energy consumption1 / GR–16-2 Eurodrip / 37.39 / 7 / AGRODRIP / 23.80
2 / GR 20–2 Eurodrip / 29.32 / 8 / TUFFTIF BLACK-2, Plastro / 34.00
3 / GR 20–4 Eurodrip / 12.73 / 9 / TUFFTIF BLACK–4, Plastro / 14.42
4 / GR 16–4 Eurodrip / 11.48 / 10 / TUFFTIF GREEN–8, Plastro / 5.52
5 / DLK02–1 TORO Ag (USA) / 39.08 / 11 / Hydrodrip II, 2, 3, Plastro / 30.6
6 / DLK04–1 TORO Ag (USA) / 17.00 / 12 / IHM–KP–4, 6 (BG) / 11.70
Treatment of irrigation water is another problem occurring when consumption of pump energy is eliminated (minimized). Pressure filters are applied traditionally - sand-gravel and gauze (disc) - where hydraulic (energy) losses are ranging most frequently from 4 up to 10 m and upstream operating head not lower than 20-25 m is required. There are two methods to solve that problem:
Increasing the cross section of dripper’s operating canal using nozzles with varying (modulus) hydraulic parameters along the length of irrigation laterals;
Using pressure-free (or low-pressure) treatment plant - open type. Small compensating basins or gravity settling basins built up for this purpose especially could be used. Gauze filters could be placed on their outlets. Since pressure upstream these filters is rather low the required flow rate could be obtained increasing their operating area. A system of coarser trash-racks could be installed upstream the gauze filters. In such a case cleaning these trash-racks shall impose further problem with a view to prevent eventual back watering and overflowing of water.
Siphon water intakes with gauze filters built in them could also be used where the water source is an open canal or tank.
Another opportunity to bring the operating pressure in irrigation laterals to the minimum and consequent elimination of the necessity to use pumps is to reduce the flow rate in drippers at the expense of increased time of water application for 24-hours (values are reciprocal) - almost continuous irrigation of 18 to 20 h. The problem arising is whether that minimum dripping flow rate will be sufficient to gain the required minimum sizes of moistening contours in depth of soil and in the same time to compensate the evapotranspiration of plants.
Not in the last place should be noted that gaining coefficient of irregular distribution of irrigation water lower than 10% is not always justifiable from economic point of view. It is required and proved for conditions only with lasting lack of rainfalls and where the water application system is the only water source and the soil possesses low water retaining capacity. In the conditions in Republic of Bulgaria and the greater part of the European region that coefficient could have higher values since any heavier rainfall would compensate the effect of irregular water distribution on yields. This allows application of drippers with lower energy consumption as shown in the above table.
In conclusion could be noted that a balanced engineering decision providing for minimum operating head of 0.5 to 1.5 m without usage of pumps and energy in irrigation laterals could be applied for dripping irrigation of small farm plots where open water sources are available with certain command position in relation to the irrigated area. Unfortunately farmers in most cases, under the growing advertising activity of companies manufacturing dripping irrigation systems and accessories, are buying complete sets of systems - pumps, filter, irrigation laterals - with practically unusable potential length - 300 to 400 meters. This raises the cost of the installation per decare and energy consumption especially if private yards and plots are to be irrigated.
There also are other possibilities and approaches to achieve that purpose depending on the flexibility of skill of designers and the available situation.
[1] Bulgaria, Sofia 1734, 1 Hristo Smirnenski St., Chief Assistant, University of Architecture, Civil Engineering and Geodesy, Hydrotechnical Faculty, Department of Irrigation and Drenage Engineering