Multi-channel estuaries, such as the Mekong Delta in Vietnam and the Scheldt in the Netherlands, have characteristics of both the river and the sea, forming a unique environment influenced by tidal movements of the sea and freshwater flow of the river. This study addresses a number of knowledge gaps in multi-channel estuaries by developing a predictive analytical approach for salinity intrusion and discharge estimate in multi-channel estuaries. The new approach agrees well with 1-D hydrodynamic models and observations, indicating its applicability in practice. Most importantly, the study has successfully developed a new theory and a new equation to quantify tidal pumping due to ebb-flood channel residual circulation and the related salt dispersion.

Salinity affects 10 to 16% of all irrigated lands while the annual rate of land loss due to waterlogging and salinity is about 0.5 million hectares per year. In this dissertation, the role of subsurface drainage to reduce these problems in irrigated agriculture in arid and semi-arid regions has been analysed and challenges for improving subsurface drainage practices have been formulated.

Although the installed subsurface drainage systems are in general technically sound and cost-effective, drainage development lags behind irrigation development and consequently a substantial part of the irrigated areas suffers from waterlogging and salinity. This is mainly because the subsurface drainage systems are designed and implemented by government, with the users, the small farmers, having little responsibility and having little input. In the adopted top-down approach the location-specific conditions and farmers? preferences are hardly taken into consideration. Furthermore, the emphasis has been on the technical aspects (the physical infrastructure), while the organizational aspects (institutional infrastructure) have been largely neglected.

To reverse the negative trend in salt build-up and waterlogging in irrigated lands in semi-arid and arid regions, a number of challenges for enhancing the role of subsurface drainage have been formulated: (i) balancing top-down against bottom-up, (ii) from standardization to flexibility and (iii) focus on capacity development.

Irrigated agriculture accounts for around 70% of global water use. However, an estimated 60% of irrigated cropland remains highly water-stressed, a problem intensified by the effects of climate change. Improving water management in agriculture considers ways of addressing this challenge. It reviews advances in monitoring and optimizing irrigation efficiency, ways of retaining and re-using water resources as well as how farmers can work collaboratively with other stakeholders to manage watersheds more sustainably. The book highlights key areas where innovation is required to ensure that water use is optimised at farm and watershed scales. The book encourages farmers to reassess their current irrigation models and implement alternative practices which improve efficiency with a reduced environmental impact.