Investigating the durability and efficiency of the subsurface drip irrigation system in sugarcane fields in southern Ahvaz

Introduction
Since sugarcane has a high-water requirement and is sensitive to waterlogging caused by rising water tables. Therefore, its irrigation requires a properly designed and implemented irrigation and drainage network. Achieving optimal irrigation and drainage conditions is often constrained by limitations in the design and implementation of the system, as well as by the availability and performance of equipment and materials. Consequently, selecting appropriate irrigation methods for sugarcane production is essential for optimizing water and energy use. Given the characteristics of subsurface drip irrigation, this method has the potential to be successful when appropriate conditions are provided, including proper hydraulic design of the irrigation network, adequate filtration of the irrigation water, and correct installation and implementation of the system. Therefore, this study was conducted to investigate dripper clogging, as well as durability and efficiency of a subsurface drip irrigation system, over a five-year period in sugarcane fields in southern Ahvaz.
 
Materials and Methods
 In this study, changes in dripper clogging  and sugarcane productivity over different cropping years were investigated in a drip irrigation system at the research station of Khuzestan Sugarcane Research and Training Institute. The treatments included: (1) subsurface drip irrigation with a dripline installation depth of 20 cm, a dripper spacing of 50 cm, and a discharge rate of 2.4 l.h^-1; (2) subsurface drip irrigation with a dripline installation depth of 20 cm, a dripper spacing of 60 cm, and a discharge rate  of 3.6 l.h^-1; (3) surface drip irrigation with a dripper spacing of 60 cm and a discharge rate of 3.6 l.h^-1; and (4) furrow irrigation as the control treatment. The drip irrigation laterals were installed before planting. Following planting, the first irrigation was performed using the subsurface drip irrigation system, and this irrigation method was continued throughout the growing season. At the end of each cropping season, four 10-meter-long sampling sections were selected from each treatment and manually harvested. Quantitative and qualitative characteristics were then measured to determine sugarcane and sugar yields (t. ha^-1). Considering the total volume of water applied during the sugarcane growing period, including irrigation and rainfall, water productivity was calculated as the ratio of yield to the total volume of water consumed.
 
Results and discussion
The results showed that irrigation water productivity decreased almost linearly over the five-year study period. The highest water productivity was recorded for subsurface drip irrigation with a dripper discharge rate of 2.4 l. ha^-1 and a dripper spacing of 50 cm spacing in the first year (6.6 kg. m^-3). The lowest water productivity was observed under surface drip irrigation with a discharge rate of 3.6 l. ha^-1 and a dripper spacing of 60 cm in the fifth year, averaging approximately 1 kg. m^-3. The results also showed that dripper clogging increased from 8.1% in the first cropping season to 31.3% in the fifth cropping season, while the distribution uniformity of the system decreased from 90% to 45%. The main cause of dripper clogging appeared to be physical clogging by very fine particles of clay, silt, and organic matter that passed through the filtration system. Acid washing and chlorination generally have limited effectiveness against this type of clogging. These particles accumulated in the narrow flow passages of the drippers, reducing their discharge and consequently decreasing uniformity of water distribution. Considering the quality of the irrigation water, the results suggest that the main problem was associated with inadequate filtration performance and insufficient control of physical clogging.
 
Conclusion and recommendations
The five-year average results showed that, compared with conventional irrigation, the subsurface drip irrigation system reduced water consumption by approximately 40% while increasing average yield by more than 25%. Consequently, water productivity increased by more than 50% for both sugarcane and sugar production. However, despite periodic acid washing and chlorination, dripper clogging increased to 31% and system distribution uniformity decreased to 45% over the five-year operating period. This deterioration was mainly attributed to the poor quality of the irrigation water and the reduced efficiency of the filtration system. The predominant type of dripper clogging was physical clogging caused by very fine particles of clay, silt, and organic matter passing through the filters and accumulating in the narrow flow passages of the drippers. Acid washing and chlorination generally had limited effectiveness in removing this type of clogging. Therefore, despite the advantages of subsurface drip irrigation in reducing water consumption and increasing yield and water productivity, the high level of dripper clogging and the substantial decline in system uniformity, together with the high initial and maintenance costs of the system, indicate that the system did not achieve the expected level of durability and operational efficiency under the conditions of this study.