Adaptation and development trade-offs: fluvial sediment deposition and the sustainability of rice-cropping in An Giang Province, Mekong Delta

Abstract

Deltas around the globe are facing a multitude of intensifying environmental change and development-linked pressures. One key concern is the reduction in the quantity of suspended sediment reaching and building floodplains. Sediment deposition provides multiple services to deltaic social-ecological systems, in particular, countering the subsidence of the delta-body, and providing plentiful nutrients. Experiencing particularly rapid change is the Vietnamese Mekong Delta (VMD). In An Giang Province an increasing number of high dyke rings, which exclude the flood and facilitate triple rice-cropping, simultaneously prevent much of the sediment load from reaching the floodplain. This paper explores the trade-offs implicit in the decision to shift from (i) doublecropping (higher sediment deposition) to (ii) triple cropping (lower sediment deposition) by asking: what is the impact of the shift on VMD farmers? Is it sustainable? And what is the significance of the associated sediment exclusion? A novel survey of An Giang rice farmers was conducted, investigating key agricultural practices, and uniquely, the farmers' estimates of annual sediment deposition depth. The survey elicits some key changes under the adapted system (ii), particularly, unsustainable trajectories in the yield to fertiliser ratio which penalise land-poor farmers. Furthermore, the value (to farmers) of the sediment contribution to agricultural fertilisation which is lost due to triple-cropping is estimated at USD 15 (±5) million annually. We argue that our growing understanding of the importance of sediment in the deltaic social-ecological system may be revealing an emergent risk; arising from conflicting long and short-term adaptation and agricultural development objectives.

Sustainable rice cultivation in the deep flooded zones of the Vietnamese Mekong Delta

“…2) made by local stakeholders about the full-dyke system, and highlights how triple-rice-cropping inside the full-dyke rings threatens the ability to meet policy objectives set out by the national and provincial authorities. Specifically, we have shown that the consequence of a full-dyke system is to reduce the depth of nutrient-rich sediment deposited on rice paddies by a factor of five [10].…”

“…Our model projections compared outcomes from a range of alternative cropping policies against the current dominant strategy of uninterrupted triplecropping [10]. These alternate policies (Fig.…”

Barriers to Implementing Irrigation and Drainage Policies in An Giang Province, Mekong Delta, Vietnam

“…Small sluices are usually at the head of secondary or tertiary canals to control water flow inside the high‐dike compartments. Because of the area's flat topography, these sluice gates are designed to distribute floodwater to the fields during the flood season (Chapman et al ., ). According to the province's regulation, each high‐dike compartment should have two small sluice gates for flooding rice fields during the third season of the third year, following the 3–3–2 cycle.…”

“…76 was issued, requiring farmers to allow their rice lands to flood in the third season of the third year. This is the only existing irrigation and drainage strategy that aims to maintain controlled flood inundation and sediment deposition in the compartments protected by high dikes (Chapman et al ., ). However, the rule has not been widely or effectively implemented by farmers and local authorities.…”

“…However, the rule has not been widely or effectively implemented by farmers and local authorities. In 2016, for example, Chapman et al ., () reported that only two of the nine communes they surveyed in An Giang were operating the 3–3–2 system. The other seven communes ignored the 3–3–2 system to practise triple‐rice cropping inside the high dikes over many years.…”