Evaluating the Hydraulic Characteristics of Gravity Subsurface Irrigation System in Sloping Laterals

Background and Objectives
Much of high-quality available water is consumed for different plants irrigation, in agricultural applications, urban green spaces and other kind of orchards. So efficient use of water resources in arid and semi-arid regions like Iran has become unavoidable strategy. Although there has been a continuous trend towards application of efficient irrigation systems such as pressurized drip irrigation methods, due to their surface evaporating, dripper clogging, complexity and high initial investment, they are not suitable for arid climates, saline water, and small scale owner applications. Thus implementation of a simple, user-friendly, subsurface, porous pipe, and gravity (pressuereless) irrigation systems is attractive for agricultural and other applications. Perforated pipes covered by plastic textile sleeves as subsurface porous emitters have been used before for irrigating the Pistachio orchards in Iran. Later on, hydraulic properties of such irrigation system has been measured and reported for flat laterals, and here the feasibility and hydraulic characteristics of this irrigating system for sloping laterals is explained. In the case of sloping lateral, if the inlet water flow rate is low, the beginning part of the lateral will be irrigated more than mid and end parts and vice versa, consequently the discharge distribution uniformity will not be attained. The solution is implementation of a step like plateaus piping system and siphon connections for each lateral. The objectives of this research are feasibility, performance and field study of hydraulic characteristics of such irrigation system in sloping lands.
Methodology
In this research, such siphon–step like piping subsurface irrigation system has been used for long (80 up to 115 meters) and sloping laterals. The subsurface porous pipe had 4 orifices per meter and its outer diameter was 63 mm. The average height of siphon barriers was 14cm. The speed of advancing waterfront was measured for different inlet water flow rates and lateral lengths. In field experiments, the hydraulic parameters including local head have been measured versus irrigation time in serial plateaus for different inlet water flow rates in the range of 0.735-1.225 lit/s and time of year. The leakage rate of individual plateaus have been evaluated using a physical model for leakage-head relation.
Findings
Practically, plateaus and siphon-like barriers increase the pressure potential energy and so result in desired irrigation distribution uniformity along the lateral. It was showed that, the speed of waterfront depends on inlet flow rate, number of siphon like barriers and their height and so lateral length. Moreover, an increase of a single barrier height causes significant delay in lateral filling and irrigation non-uniformity. According to aforementioned physical model, the discharge rate of such porous pipe is proportion to water head and orifices characteristics. Thus, the infiltration rate of the plateaus and the orifice parameter have been evaluated for different inlet water flow rates and lateral lengths. The simulation of leakage rate of plateaus provides a measure of irrigation distribution uniformity along the laterals. It is revealed that the irrigation uniformity can be achieved provided the inlet water flux to be set carefully (more than the optimum value) proportional to lateral length and time of year. As the leakage rate of this subsurface, gravity irrigation system is much more than other subsurface porous pipes, thus it can be a promising technique for agricultural irrigation in arid climates and saline water resources.
Conclusion
Water shortage and low efficient use of agricultural water in arid and semi-arid regions is increasingly becoming a global concern. Thus implementation of low pressure (gravity) subsurface irrigation method can be a solution for small scale owners and saline water resources. Perforated pipe covered by a plastic textile sleeve, can be used as a high hydraulic conductivity porous pipe for this task. The irrigated water distribution uniformity is a matter of concern in sloping laterals than flat ones, and a step like plateaus piping system with siphon connections has been used as a practical solution. Indeed, plateaus and siphon-like barriers increase the pressure potential energy and so result in irrigation distribution uniformity along the lateral. In field experiments, the speed of waterfront and the hydraulic parameters including local head, have been measured over time in serial plateaus for different inlet water flow rates and lateral lengths. The leakage rate of individual plateaus have been evaluated using a physical leakage-head relation. It is revealed that the desired irrigation uniformity can be achieved provided the inlet water flux to be set carefully (more than the optimum value) proportional to lateral length and time of year. This simple, user-friendly, gravity, subsurface irrigation system can be a practical irrigation system for agricultural applications in arid regions, saline water resources and sloping laterals.