1. Introduction
Cambodia, one of the most vulnerable countries in Southeast Asia to climate change [
1,
2], will likely experience higher precipitation, increased seasonality and increasing temperatures under the changing climate [
3,
4]. With a monsoon-driven tropical climate, Cambodia already experiences many types of hydro-meteorological hazards including droughts, heavy storms and both flash and prolonged flooding, which may become more extreme under climate change [
5]. It is not just the climatic impacts that makes Cambodia vulnerable, but people’s reliance on climate-sensitive activities and the challenges faced in water management. People in Cambodia rely heavily on rice and fish for food while rice production, fishing and forestry employ 70% of the labor force [
6,
7]. Rice, specifically, is a major consideration in the national food security effort [
8,
9], ever more important in a country where 26% of the population remains malnourished [
2]. Rice production already faces the challenges of floods and droughts and rice farmers have expressed concerns over their ability to adapt to climate change, reporting adverse field conditions under higher temperatures and less rainfall and subsequent reductions in yields [
10,
11].
While climate change poses an important risk to the country’s natural resources, agricultural expansion continues to be an important economic goal and the practices and infrastructure to achieve this may in fact provide climate adaptation strategies [
12,
13]. Modern rice varieties with shorter rice cultivation periods can provide surplus production beyond the traditional wet season cultivation [
14]. This practice of double-cropping has gradually increased in Cambodia since the introduction of short- and medium-duration rice varieties [
15]. Multiple cropping can not only ensure subsistence for rice-growing households but can provide an additional source of income and can allow for flexibility in response to erratic changes in rainfall patterns [
16].
Despite progress made in increasing the number of rice plantings per year, Cambodia suffers from low rice yields relative to other rice-producing countries in the region [
17] due to a variety of reasons, but most notably due to poor water management related to too much or too little water [
9,
12]. Cambodia’s climate of extremes—too much water in the wet period, and insufficient in the dry period—makes water management essential for the continued development of agriculture and adaptation to climate change [
18]. While double-cropping has increased during the wet season, less than 10% of rice is grown during the dry season due to limited water availability, lack of irrigation systems and insufficient reservoir storage [
14,
17]. A study in the Pursat River Basin suggests that reservoir development can be one of the water management options to adapt to the expected significant declines in future water availability [
11]. However, reservoir development options cannot be contemplated in isolation from the upstream activities related to land use changes [
19], the increasing water infrastructure development in the Mekong watershed and impacts on flow regimes and aquatic species [
20]. Recent studies suggest that adaptation options need to be evaluated at the community level, focusing on improving irrigation technologies and community-level monitoring systems [
10].
Community-level irrigation infrastructure is not only required to make dry season rice production possible but can also provide added water security and reliability in a changing climate [
16]. Murphy et al. indicates that infrastructure development and less water-intensive methods for rice production can contribute to the reliability of the system and the reduction of greenhouse gas emissions from rice [
21]. This research emphasizes the notion that community-based environmental management can be a key intervention to counteract the “perceived inadequacies of top-down or central government management” [
22]. Technical models can provide the tools to scale-up community-based actions to examine the implications of those actions at the watershed level, which can in turn contribute to the coordination between water users, but this would require multiscale water governance, another area where Cambodia faces challenges.
Recent changes in water governance have caused fragmented decision-making processes and left the system dysfunctional in places [
7]. Irrigation supply relies on Farmer Water User Communities (FWUCs), which are local government entities made up of farmers with limited resources, power or capacity. These challenges limit farmers’ independence and ability to respond with large-scale adaptation measures, such as growing dry season rice [
23]. A number of studies have shown that increased fertilization and irrigation are important climate change adaptation actions available to farmers [
24,
25]. However, watershed level impacts on the hydrology need to be evaluated—in particular, impacts on the downstream parts of the watershed and the potential tradeoffs between irrigated agriculture and required flows for fish habitats [
24,
25]. The very irrigation practices, fertilization and multiple cropping that may make rice farming less vulnerable against climate change may have the opposite effect on fish populations. Stresses on water supply from climate change and increases in demands from rice production goals will require better management of water resources to ensure sustainability of livelihoods and ecosystems.
Cambodia is currently faced with many challenges, from extreme weather events to low rice yields, desires to increase rice production but a lack of water management to do so effectively and a population that relies on the health of agriculture and fisheries to survive. Climate change will likely exacerbate all these challenges. While many studies have been conducted to understand the potential impacts of climate change on water availability and hydrology [
18,
26,
27,
28,
29], how and to what extent rice production can reliably and sustainably expand under climate change is still largely uncertain. This study aims to better understand the interconnectedness of climate change, rice production and potential impacts of both to downstream flows required for fish habitat and other ecosystem services. We provide a watershed-level analysis of climate change impacts and water management implications for people, food security and the sustainability of ecosystem services in the Stung Chinit watershed by investigating the tradeoffs between different strategies of increasing rice production and protecting environmental flows. The Stung Chinit is a watershed where community-managed water resource systems are grappling with the complexities of conflicting objectives of producing more rice and supporting fishing activities while they are developing their water governance and infrastructure under a changing climate. This is a challenge that is shared by other watersheds in the region and worldwide. We first describe the Stung Chinit watershed and the specific challenges which it faces relevant to climate change and water management, we then describe the model developed to conduct the analysis, the analysis itself and, finally, findings and discussion.
2. Study Area
The Stung Chinit, at approximately 264 km long with a watershed of approximately 8236 km
2, is located primarily in Kampong Thom Province in Cambodia (
Figure 1). The river is a major tributary of the Tonle Sap Lake (
Figure 1), the largest and most important lake in Cambodia both in terms of economy and water supply, supporting one of the world’s most productive ecosystems [
30]. Like the rest of Cambodia, rice production and fishing are essential to the lives and livelihoods of those living in the basin, but at times, these objectives may be at odds. Large-scale global changes such as hydropower development, climate change and urban migration are expected to negatively impact individuals who depend on fisheries of the Tonle Sap [
31]. This watershed has been found to be particularly vulnerable to climate extremes, considering different areas in Cambodia [
18].
Approximately 90% of cropland in the Stung Chinit watershed is rice, with the majority of rice cultivation occurring in the wet season [
17]. While irrigation schemes exist within the basin, most of the rice is rainfed and multiple cropping is still uncommon [
14]. After the development of irrigation schemes in the Stung Chinit watershed, villages downstream noted declines in fish size and populations in the dry seasons due to lower than usual flows, with water levels that limit their ability to travel by boat [
18].
In the Stung Chinit watershed, there are two main reservoirs as well as multiple smaller reservoirs and ponds, which supply irrigation water to farmers’ fields through a system of primary, secondary and tertiary canals. The two main reservoirs, the Stung Chinit and Taing Krasaing (
Figure 1), and their primary canals, though built by the national government, are now managed by the Provincial Department of Water Resources and Meteorology (PDoWRAM), and the secondary and tertiary canals are managed by the Farmer Water User Communities (FWUCs) [
23,
32].
FWUCs may operate in different ways. In the Stung Chinit FWUC, which manages canals in one of the larger irrigation schemes, irrigation scheduling is done in coordination with farmers and the Chinit River Irrigation Committee, and farmers are included in all steps of the irrigation planning [
32]. This method seems to be operating well, as the majority (75%) of farmers indicated that they receive sufficient water at the right time [
32]. Those with access to water from the canals have seen improved livelihoods due to the development of the irrigation scheme, as they can plant rice earlier, more reliably and in the dry season [
18].
A site visit to the Taing Krasaing FWUC demonstrated that schedules are set more ad hoc, based on rain events, but timing of releases is communicated to farmers, and farmers can request additional water when needed. (As part of this study, three workshops and one site visit to the two reservoirs in the system were conducted between April and December of 2018. As it pertains to this work, the workshops served to give stakeholders an opportunity to give input and learn from the model development and analysis. Participants included members of the PDoWRAM, Provincial Department of Agriculture and Department of Environment, Commune and District government leaders and FWUC, forestry and fishery community leaders and members.) Among other FWUCs, some may develop set schedules for the release of irrigation water into canals, some may have an allocation method but may not discuss it with farmers, and others still may operate on an ad hoc basis with little planning and coordination, only operating based on daily needs [
33]. Overall, cooperation and management within the basin is found to be lacking the capacity to respond to the threats which climate change poses [
18].
6. Conclusions
Due to the relative demands for streamflows and rice in the dry season, if dry season rice planting is widely promoted, this will severely impact streamflows. However, implementing a flow requirement protects these flows, while only causing minor shortages to rice when the wet season growing period is extended by planting rice once or twice per year. These shortages may be alleviated with improved cooperation and management in the river basin and shifting rice irrigation practices. While climate change is a threat, and will lead to warming temperatures and therefore potentially higher demands for rice irrigation, the larger threat to rice and ecosystems appear to be the management (or lack thereof) of the system to prioritize water uses. While this study suggests that there is physically sufficient water to expand the irrigated area, grow rice twice per year and protect downstream flows, this water may not be available to all, and significant management is required to ensure this, which is lacking. It will be important to better understand who can and cannot grow rice multiple times per year, due to impacts beyond water availability. As this study focused largely on availability; management, poverty and other barriers to access may be preventing this system from reaching its full potential, both in protecting ecosystems and providing resilient livelihoods for its people.