Insights on the irrigation conveyance efficiency of three canal systems for improving agricultural sustainability in Sri Lanka Open Access

Water is a critical resource for sustaining life on Earth, playing an essential role in ecosystems, agriculture, industry, and energy production (De Fraiture et al. 2010; Cosgrove & Loucks 2015; Usman & Radulescu 2022). In agriculture, water is particularly crucial for irrigation, where it directly influences crop growth and yield (Malakar et al. 2019). As the global population increases, the need for efficient water resource management in irrigation becomes paramount to ensure food security and sustainable agricultural productivity (Zarghaami 2006). Modern irrigation systems aim to optimize water use efficiency, reduce the impact of droughts, and enhance livelihood support, especially in regions reliant on agriculture. However, the sustainability of these systems heavily depends on conveyance efficiencies – the proportion of water delivered to the crops compared to the amount released from the source (Stout 1999).

Surface irrigation systems, which are common in many parts of the world, often suffer from low conveyance efficiencies, with significant water losses due to seepage, deep percolation, and surface runoff. Studies suggest that approximately 60% of water can be lost during distribution at the field level (Evans & Sadler 2008; Ali 2010; De Pascale et al. 2011). Large-scale irrigation systems, such as those in Sri Lanka, often experience these losses, which may be irrecoverable when water escapes the system and enters natural drainage ways or percolates beyond the root zone (Gleick 2003; Meijer et al. 2006). Factors like over-irrigation, poor canal management, and soil porosity significantly affect irrigation efficiency at various stages of water conveyance (Kumar et al. 2008).

Water conservation and optimal use are achieved through efficient conveyance, which also minimizes the negative environmental effects of excessive water withdrawals and lowers energy consumption through water conservation (Hamdy et al. 2003). A number of practices have been used to improve the efficiency of irrigation conveyance systems. These practices include the use of innovative irrigation techniques like lined canals and water management strategies designed to minimize seepage and evaporation losses, the application of precision agriculture technologies for optimal water delivery, and the integration of data analytics and remote sensing for monitoring and management in real-time (Rezapour Tabari et al. 2014; Adeyemi et al. 2017; Calera et al. 2017; Ashour et al. 2023). Rehabilitation and modernization of ageing canal networks, the implementation of water-saving techniques, and institutional reforms to improve governance and water allocation are widely used in south Asian countries due to their high agricultural populations (Sathre et al. 2022). Adopting these strategies not only boosts agricultural productivity but also promotes sustainable water management, climate resilience, and socioeconomic development in both the global and South Asian contexts (Cai et al. 2015).

Rapid population growth in the 20th century in Sri Lanka caused severe stress on food production. Therefore, the irrigatable areas are significantly increased (Marambe et al. 2017). Higher emphasis was shown on expanding irrigable rice areas since irrigated paddy gives a higher assurance of rice production. During that period, new irrigation areas were developed such as Mahaweli, Walawe, and Kirindi Oya river basins while existing irrigation areas such as Parakrama Samudra and Kantale were also rehabilitated and renovated. Interestingly, the irrigation distribution canal network consisting of main, distributary, and field canals of almost all these irrigation areas are made of earthen canals with earthen lining except for a few trans-basin canals which consist of concrete lining (Wijesekara 2011; El-Molla & El-Molla 2021).

Various technological approaches have been adopted to increase the system efficiencies, including concrete lining, rotational irrigation, and crop diversification. Greater emphasis was laid on reducing water losses that would result in increased irrigation efficiencies. However, limited research has been conducted on system irrigation efficiencies in the context of Sri Lanka (Gnanadasa 2010). At present, irrigation authorities use a standard set of guidelines for irrigation efficiencies as documented by Ponrajah (1989). These guidelines are given in general terms to obtain an approximate judgment of the irrigation needs of paddy cultivation during a dry period. It does not give an indication of the boundary conditions such as the efficiency value when conveying under saturated conditions or under various soil physical conditions. Therefore, when the canals are planned, the cost estimates are done assuming the soil characteristics are uniform throughout the canal trace (Swamee et al. 2000; Haymale et al. 2020).

On the contrary, the porosity of the soil differs frequently with varying textural characteristics; thus, it requires adopting different lining methods (Yao et al. 2012; Han et al. 2020). This is important in a situation where funds are limited for irrigation projects in most of the developing countries. The decision has to be made for the choice between full or partial concrete and earthen lining or piped conveyance based on the available funds. Therefore, in situ investigation of water losses along the canal due to soil textural variations may help in planning the lining of canals. The earthen lining is usually preferred for a clayey texture whereas concrete lining would be required for a sandy porous texture (Martin 1980; Zhang et al. 2017). Therefore, the lining material can be decided as required on location. This is important in the context of the cost evaluation since it would reduce unnecessary expenditure (Radcliffe et al. 2002).

Therefore, this study was conducted to ascertain whether there is a deviation of irrigation efficiencies from the guidelines given by Ponrajah (1989) and whether there are differences in water loss rates along the same canal owing to soil textural differences. Ponrajah's guidelines emphasize minimizing water losses and optimizing irrigation efficiencies, which he defined as typically ranging between 60 and 80% in tropical regions like Sri Lanka. These recommendations include best practices for canal design, such as proper dimensions, slope, and lining material, and highlight the impact of soil texture on water loss rates, with clay soils reducing seepage and sandy soils increasing it. This study is the first since 1989 to evaluate irrigation efficiencies in distributary and field canals in Sri Lanka and aims to reassess and potentially revise these standards to address modern challenges. By estimating conveyance losses, the research offers insights into deviations from established guidelines, thereby advancing sustainable water management and providing practical recommendations for future canal planning and rehabilitation.

The main canals leading from the sluices are unlined. Left Bank Main Canal consists of nine distributary canals and 21 field canals while Right Bank Main Canal consists of two branch canals, eight distributary canals, and 25 field canals. Figure 3(b) depicts the selected locations for the study.

A field reconnaissance survey was carried out to select suitable watercourses for study, observe field conditions with regards to usage of equipment for flow measurements and implementation methods, inspection path availability, and analyze the impacts of waterlogging in the study area.

In this study, the mentioned locations were used as head, middle, and tail sections of all three field canals of all three locations. Field tests were carried out to select suitable watercourses for study, to observe the field conditions by considering the equipment needed to measure the infiltrated depths, to find the availability to inspect the location, and to analyze the impacts of waterlogging in the study area. However, field testing was delayed in water issuing for the Maha season (2022/2023) because of issues that occurred in fertilizer. In that case, tests were done only in a few selected canals of PDAs. Furthermore, during the data acquisition process, factors such as flow turbidity deteriorated and non-geometric sections, sedimentation, and vegetation in the watercourses affected the results of some of the field canals as a result, some of the points in the head, middle, and tail at these canals were omitted and locations free of difficulties when measuring losses were considered in analyzing data.

On-site coordinates of watercourses and reference points were collected using the global positioning system. The collected coordinates were then validated through Google Earth Pro Software. The validated image file was further processed in ArcGIS software to digitize and map the spatial data.

The field surveys were designed to check the accuracy of the secondary data and to collect the primary data by filling in the data gaps. The field data were used directly in this study. Due to the lack of high-resolution soil maps at the pilot level, there was a need to conduct the soil survey manually. Soil sampling was carried out using soil augers and the relevant soil types, texture, and drainage were identified with geo-tagged sampling locations. This soil survey is suitable to conduct immediately after the harvesting since the fields are clean-weeded and also the soil condition is in ideal semi-dry status to the auger. Canal water flow measurements that help to ascertain system efficiencies need to be conducted when the soil profile is saturated and in an equilibrium state.

Infiltration rate, a crucial parameter for understanding water movement within soil profiles, was assessed to complement the soil survey conducted in the study area. Drawing insights from Mohammed & Sayl (2021), a GIS-based multicriteria decision approach was adopted to analyze groundwater potential zones in the Western Desert of Iraq. This methodology, integrating various factors influencing groundwater occurrence, offers valuable insights into soil characteristics and their implications for infiltration rates. By leveraging this approach, the study gained a deeper understanding of soil permeability and its impact on water infiltration, aiding in the assessment of water management strategies and agricultural practices within the region.

However, during the data acquisition process, factors such as flow turbidity, deteriorated/non-geometric sections, sedimentation, and vegetation in the watercourses provided obstacles in the flow measurement process. As a result, some of the measured/recorded data were omitted and channel sections free of obstacles were included in the result analysis phase.

The conveyance losses in an irrigation system occur due to seepage, percolation, leakages at structures in the canals, and poor maintenance. Conveyance efficiency is the ratio of the amount of water supplied at the farm level to the amount of water released from the reservoir or any control gate. Hence in order to compute the conveyance efficiency, it is required to find the conveyance loss of the canal system. Conveyance losses are affected by a number of factors such as depth of water, quality of canal lining, canal dimensions, bed slope, volume of sediment, age of canal, volume of flow, velocity of flow, and the quality of operations and maintenance of water canals (Mohammadi et al. 2019; Syed et al. 2021). Conveyance efficiencies in this study were computed by subtracting obtained conveyance losses at each reference point from 100 (Shah et al. 2021). In addition, evaporation losses were not considered due to insignificant numbers compared with conveyance losses and infiltration losses.

Flow measurements were carried out in four field canals for the estimation of conveyance losses. The selected canals are FC 64, FC 70, and SFC1/FC67 from the right bank and FC 63 in the left bank area, as shown in Figure 2(b). In each canal, the discharges were taken at three locations: head, middle, and tail end of the canal. Table 1 gives the conveyance losses in each segment of the selected canals and the lined lengths.

The high conveyance losses in the Maduru Oya scheme, averaging 28%, can be attributed to a complex interplay of factors. Despite the partial lining of canals, losses remain significant due to varying soil compositions, with sandy and gravelly soils contributing to higher infiltration rates. The effectiveness of canal lining is questionable in some sections, suggesting potential issues with lining quality or maintenance. Topographical factors, such as proximity to drainage canals and elevation differences, exacerbate seepage in certain areas. Additionally, the inconsistent relationship between lining percentage and losses indicates that other factors like groundwater interactions, canal design, and construction quality play crucial roles. These multiple variables contribute to the overall high conveyance losses observed in the system, highlighting the need for a comprehensive approach to improve irrigation efficiency.

The conveyance losses computed for the above canals were further analyzed using a histogram to obtain an average conveyance loss (refer to Figure 7(b)). A weighted average was computed for the pilot demonstration area of the Maduru Oya scheme. An average value of 28% was obtained and this value is relatively higher for partly lined canals.

Flow measurements were taken only at two locations of FC1 to cover one segment in the middle section of the canal as the head end. The tail-end canal sections were unable to be accessed as the operation and maintenance road was covered with vegetation. Table 2 gives the details of conveyance loss measurement segments and the conveyance losses in each segment. Very high losses are shown at the tail end of FC3 and FC7, whereas very low conveyance losses can be seen in the D3 canal at the initial and middle reaches. Overall, conveyance loss in the D3 canal is comparatively low due to its topography, where the canal has a deep-cut section that minimizes seepage.

The conveyance losses in the Huruluwewa LB D3 canal system exhibit significant variability, ranging from very low losses in the D3 canal to extremely high losses in the tail ends of FC3 and FC7.

This variation can be attributed to factors such as soil composition (high losses in some field canals of FC3 and FC7 values such as up to 60%), likely gravelly clay with higher infiltration rates, and potentially inadequate or deteriorating lining. The overall average conveyance loss of 26.5% (or 16.9% excluding extreme values) indicates a system with moderate efficiency, influenced by varying canal conditions. The high standard deviation (20.4%) further underscores the system's inconsistency, pointing to the need for targeted improvements in high-loss areas.

Table 3 presents the details of conveyance loss measurement segments and the conveyance losses in each segment for the Minipe D47 canal. The lowest conveyance loss percentage of 12.7% was in segment 2 of the D47 canal whereas the highest loss percentage of 56.8% was at the tail end of the D47/LB canal. It can be seen from the results that conveyance loss in the tail end of each canal is higher than the head end losses. The lowest average percolation loss was observed at the Mailapitiya canal whereas the highest was in the D47/LB canal.

The Minipe D47 canal system shows considerable variation in losses, with an average of 33.1% and a high standard deviation of 15.4%. Losses range from 12.7% in segment 2 to 56.8% at the D47/LB tail end. The consistent pattern of higher losses at canal tail ends suggests cumulative effects of seepage and potentially reduced maintenance in these areas. The predominant sandy clay soil, with its high-infiltration rate (20–30 mm/h), significantly contributes to losses. Limited canal lining (28% of total length) is insufficient to control seepage, particularly in high-infiltration areas. Histogram analysis shows most values fall between 29 and 43%, with an overall average of 34.4%, indicating moderately high losses. This highlights the need for targeted interventions, such as increased lining, improved maintenance, and possible redesign of high-loss sections.

The conveyance losses in the Minipe D32 canal (Stage II) exhibit significant variability, with an average loss of 27.7% and a high standard deviation of 23.7%. Losses are lowest in segments with higher lining (14.3 and 13.8% in segments 1 and 2) and highest in unlined sections (55.1% in segment 3). The sandy clay soil type contributes to seepage, but the effectiveness of lining in segment 2 (highest lined percentage about 36%) demonstrates its importance in loss reduction. The stark increase in losses in unlined sections emphasizes the need for targeted infrastructure improvements and policy changes to promote water conservation and irrigation efficiency.

Assessing the conveyance losses is always important to enhance the efficiency of the irrigation systems. This is important in the areas where the irrigation is done mainly using surface water. Enhancing the efficiency of the irrigation systems while minimizing the losses paves the path to achieving sustainable development goals (2, 12, and 13). Sri Lanka, in its post-COVID-19 development, is facing some economic issues which led the country to the international economic standards. This has adversely impacted the country wise food production eventually leading to the malnutrition of a significant population. Considering all these ongoing issues, the research presented here showcases some significant improvements that we can incorporate into the food security of the country.

On the other hand, most of the farmers of Sri Lanka do not give value to the irrigation water since it was given free of charge. Therefore, the losses are kept at a very high level. The findings of this research can be used to improve the water management sector and then ultimately to develop a country-wide water management policy.

This study assessed actual conveyance losses in three major irrigation schemes in Sri Lanka: Maduru Oya, Hurulu Wewa, and Minipe Left Bank Canal. The research revealed significant variations in losses, ranging from 20 to 38% at Maduru Oya, 9.2–60% at Hurulu Wewa, and 12.7–55.1% at Minipe. The average losses at Maduru Oya were 27.8% (SD = 6.03%), Hurulu Wewa were 26.5% (SD = 20.4%), the D47 canal was 33.1% (SD = 15.4%), and the D32 canal at Minipe Left Bank was 27.7% (SD = 23.7%). It is noteworthy that tail-end parts continuously showed larger losses, frequently up to 60%, which had a major effect on the overall efficiency of the system. The state of the canal lining, the texture of the soil (especially sandy soils), and topographical characteristics like the elevation disparities between drainage and field canals were important factors affecting these losses.

Based on these findings, the study recommends several strategies to improve irrigation efficiency:

• Canal lining and structural improvements: Conduct soil surveys and prioritize lining in porous soil areas, using partial lining where cost-effective. Retrofit or replace malfunctioning structures, install flow control systems, and consider pipelines for high-loss sections.

• Maintenance strategies: Develop tailored maintenance plans with more frequent schedules for high-loss areas and train local water user associations.

• Water management: Introduce rotational water supply schedules, promote water-saving techniques like alternate wetting and drying for paddy, and encourage crop diversification.

• Technology integration: Utilize IoT sensors for real-time monitoring, GIS for large-scale assessments, and a centralized database to track water efficiency.

• Awareness and policy reforms: Provide training for officials and farmers on water conservation, update irrigation guidelines, introduce water pricing, and incentivize efficient irrigation schemes.

• Research and innovation: Explore cost-effective lining materials, and nature-based seepage reduction solutions, and test advanced irrigation technologies.

• Watershed management: Implement integrated water management plans, promote reforestation and soil conservation, and engage stakeholders in better water governance.

• Monitoring and evaluation: Standardize efficiency assessments, conduct annual reviews, and use data-driven decision-making for resource allocation.

These targeted interventions aim to enhance water conservation, reduce conveyance losses, and ensure sustainable irrigation practices for Sri Lanka's agricultural sector.

The authors would like to sincerely thank the Mahaweli Water Security Investment Program Sri Lanka for their support in gathering data for this study.

This research received no external funding.

All relevant data are included in the paper or its Supplementary Information.

The authors declare there is no conflict.