The natural hydrologic conditions of the Chao Phraya delta have been deeply modified all along this century. With the implementation of the different phases of the Greater Chao Phraya Project, together with the construction of two main dams in the upper Chao Phraya basin (Bhumipol dam in 1968 and Sirikit dam in 1976), modern irrigation developed, allowing the adoption and expansion of High Yield Varieties (HYVs) in most of the area. The Improvement of drainage was gradually given priority only from the late sixties onward.

However, many low lying, ill-drained or flooded areas in the Central Plain are still planted with traditional rice varieties (TV), including both deep water rice - DWR - (suitable for water depths between 50 and 100 cm) and floating rice – FR - (adapted to water depths between 100 and 350 cm and provided with rapid elongation ability). The area cropped with Traditional Varieties (TV) corresponds to almost 400,000 ha (2,500,000 rai), more than half of it located in the Chao Phraya/Lop Buri flood plain where it makes up a total gross area of 300,000 ha (2 million rai).

Nowadays, however, "flooding" must not be thought of according to the old idea of river overflow but, rather, as a situation in which the main rivers - channelled between lateral dikes - show (or at least may show) high levels which impede full drainage of inner lands. These areas are protected from the rivers by dikes but, on the other hand, cannot evacuate the water coming from different sources and accumulating inside. Therefore, the solution chosen to stabilise rice cultivation in these flood-prone areas consists in regulating the rise of water in terms of increase rate, maximum level and flood duration.

Oddly enough, while there are numerous and sometimes redundant studies about irrigation and water distribution in the Chao Phraya delta, there is almost no information available on drainage regulation and flood-prone rice systems. This report presents an in-depth analysis of rice cultivation – its characteristics and trends -, and of water management in these areas. It combines information originating from field surveys (near 900 observation points, a third of them including a detailed questionnaire), satellite images, GIS, DEM (digital elevation model) and hydrological data from RID.

 Drainage boxes

A description of the functioning of drainage units ("boxes") in terms of water management is followed by a detailed inventory of all main and sub-boxes, with their respective regulators and topographical characteristics. The boxes generally include HYV on the high lands, deep-water rice in middle elevations and floating rice in the lower parts. They are bordered by dikes or irrigation canals and their drainage to the river system is controlled by regulators. The total flood plain is comprised of 18 main boxes, making up a total of 120 sub-boxes.

Water management in the boxes follows four different phases. (1) First is the crop establishment phase (rice is sown with dry-broadcasting), during which the box outlet regulator is closed to store water in the drain for dry-season cropping but no water accumulates in the fields. (2) During the box filling up phase, the regulator is normally closed to allow the accumulation of water and the steady rise of the water level; (3) when the regulation level is attained, the regulator is operated to evacuate possible excess flows; (4) when the rice is almost ripe, the box is fully drained and the rice harvested, starting with the high lands.

It is shown how diverse water inflows (rainfall, inner runoff, return flow from irrigation, inflows from rivers and, sometimes, sideflows) combine to define, together with the box topographical characteristics and the regulation water depth, the hydrological behaviour of each box, in particular their sensitivity to dry or wet years.

The Digital Elevation Models has allowed the accurate determination of the storage capacity of each box and of the whole upper-delta.

 Rice systems

Valuable information has been obtained through the survey of approximately 300,000 ha cropped with deep water and floating rice, totalling close to 900 observation points. The main features of these rice systems can be summarised as follows :

- rather good control and risk reduction provided by land development and water control devices;

- a productivity approximately 60 % of HYVs’ productivity;

- the substitution of natural fertilisation with chemical fertiliser; 72 % of the plots cropped with traditional varieties receive fertiliser (an average of 32 kg/rai, when they do);

- the common absence of on-farm structures and/or a location far from irrigation canals;

- a low or irregular frequency of double cropping, partly due to the above factors; but a trend towards increasing this frequency, with significant investments in plot improvement. This trend has been boosted by the high water allocation experienced in the last 3 years.

- a trend towards mechanisation of harvest, with 72 % of plots using mechanical harvesters;

- a reduction of the diversity of rice varieties used in the area; six main varieties make up 58 % of the TVs and, together with the next 17 main varieties, 82 % of the whole.

- 60 varieties were reported in use at present, while 80 were mentioned by farmers when asked about cultivated in the past; Out of these, 43 disappeared (were not found in the survey).

- A low level of occurrence of recommended varieties

The main yield-limiting factor is probably the risk which prevails at the time of crop establishment under rainfed conditions. Little can be done to circumvent hazards derived from irregular rainfall, apart from expanding irrigation facilities.

Regarding cropping techniques, the survey demonstrated that there is no simple correspondence between the use of TVs and crop establishment through dry broadcasting. DWR, and sometimes even FR, are established with both dry broadcasting and wet broadcasting. The latter case is found in areas with a proper irrigation system but insufficient drainage (the risk of flooding is dealt with by using TVs) and when the plot is also used for dry-season cropping (growing HYV with wet broadcasting).

The disappearance of transplanting, in full realisation in the 80's and completed in the early 90's, is also an important point : it significantly eased water management and removed a major bottleneck in terms of labour and farm activity planning. The last bottleneck, harvesting, is now dealt with through mechanisation.

 Water and flood management

The area cropped with Traditional Varieties has decreased and is now confined to a "flood-prone" area in which the water regime is largely controlled by means of dikes and regulators. Along the Chao Phraya river, for example, most of the floodways have been closed during the 70's. After the floods of 1975, the embankments have been raised 50 cm of the flood level.

Given that - except in dramatic years such as 1995 - the water level in the drainage boxes is controlled and artificially regulated, it is meaningless to speak of and derive statistics on "flood depth" : rather, attention must be focused on the spatial distribution of the drainage units ("boxes") and on the parameters of drainage regulation in each box : rate of filling up, optimal regulation level, date of gate opening, rate of box drainage, etc.

The boxes constitute off-channel reservoirs but are not 'conservation areas', like in the lower delta, because they don't store water to be later used locally. Rather, they are buffer areas, allowing the storage of excess water in the rainy season. However, it is important to understand that their main purpose is to provide adequate flooded conditions for the growth of TVs in areas where (a) the plot conditions and/or (b) the conditions of access to water and/or (c) the risk of submergence as governed by the drainage conditions, do not allow the cultivation of HYVs. This suits the need for flood relief but it must be stressed that in most years, under the prevailing water regime, such buffer function is not fully needed.

A few important findings are noteworthy :

1. During the month of October the water stock rises gradually from 40 to 100 % of the full storage capacity. When the drainage boxes attain their full storage capacity, sometimes around the 1^st of November, 2 billion m³ of water are stored. The buffer capacity of the area - its normal capacity to act as a flood relief area - decreases accordingly.

2. This stock in an average year is estimated to be more than twice the quantity of water stored in the lower delta in a year with an overall 50 cm flood in the upper half of the West Bank. In a year with no particular excess water (like in 1998), the West Bank stores an equivalent of only 5 % of the volume stored in the upper delta, mainly in its canal system.

3. The margin of box overloading is extremely significant and corresponds to an increase of 50 % in the storage capacity for an overall 25 cm hike in the water levels. The mapping of the box status at a given instant may show where and how much additional storage capacity is available. Overloading can be achieved by several waterways depending on the box (drainage regulators, irrigation canals, wasteways, sideflows, etc) and the height of the dike.

4. A monitoring "dashboard" is proposed, in order to monitor the status of the boxes and orient decision-making in case of drastic flood. It allows to pinpoint which boxes still have storage capacity and which are overloaded (and with how much water). Its establishment would require to add a few observation points of the water level in some boxes which are not monitored.

5. It does not appear than any limited reduction in the storage capacity would significantly jeopardise the flood relief function of the area, especially from the 1^st of November onward.

6. Water control in the boxes appears satisfactory, as intermediate regulators now also provides increased local control. However, in dry years, some boxes face difficulties in the filling up phase and the upper lands may lack of water.

7. The coordination of drainage within a "cascade box" pose some problems : the decision-making process on the date of gate opening (the date must be adjusted each year to some particular cases), and the congruence between water management and the choice of rice varieties are the object of discussions, and sometimes conflicts, almost every year in most of the boxes.

This points should be investigated by agronomists. It is also hypothesised that the importance of knowing the rice characteristics (cycle, height, elongation ability, etc) before adopting them in a given box could be one of the factors explaining the low level of adoption of recommended varieties in the area.

8. There is a strong and quantitative evidence of the marginal re-use of the water drained out of the drainage boxes for DS cropping in the lower delta. This is due to the fact that the boxes drainage occurs in a period in which the water demand from the conservation area is still low. At least 85 % of the water is lost to the sea. This dismisses any possible fears that a reduction in the storage volume would impact negatively on dry-season cropping in the lower delta.

9. The drainage of the boxes generates an inflow to the Chao Phraya reaching a maximum discharge of 700 cms, in the second half of December.

 Perspective of change and intensification

Several evolutions have been observed and can be extrapolated for the future. The main driving force is probably the low profitability of TV rice farming. In the long term, farmers are compelled to find some way either to intensify or to diversify rice farming, or to give up agriculture. The reports provides current examples of these trends, identifies their advantages, limits and constraints.

1. The first evolution is possible in areas where the water regime can be altered in order to accommodate HYVs instead of TVs. This has been possible in areas like Borommathad Project and amphoe Tha Wung and can be expanded to boxes like Lam Chuad or Don Tum box, or achieved on the higher land of the boxes by moderately lowering the water level in some boxes.

The transition area on the eastern side, formerly using transplanting, still harbours a lot of DWR although it is little or not prone to submergence risk and has irrigation facilities; it remains one of the rare cases of TVs grown under irrigated conditions in Asia and an in-depth investigation should be carried out in this area to assess to what extend this situation could be remedied.

2. The second path is to increase the cropping area in the dry season : a first solution would be to tap water from the Mekong or Salaween rivers in order to increase the water available in the dams. Improvements may also come 1) from improved water scheduling and distribution ; 2) secondary water sources, namely tube wells, remaining water in drains, reservoirs excavated in low lying spots. Even under the current limitations, it is advisable to achieve more equity and not systematically disregard these areas cropped with traditional varieties. Most of them are now in a condition to grow dry-season rice and the sustainability of farming strongly depends upon the frequency of dry-season cropping.

3. The third path is to abandon wet-season rice cropping and start, as early as possible, a DS crop at the end of the rainy season. Depending on water sources available locally, two rice crops can sometimes be accommodated in the dry season. If the whole box follows such a path, then there is no more scope for storing and releasing water according to the former pattern : the receding of the flood must be let to natural conditions, allowing in most years a much earlier DS cropping. There is scope to allow Phak Hai project to follow the transformation initiated by the West bank 20 years ago.

4. The last evolution path observed is the abandonment of rice farming and/or agriculture. This move has been observed most especially in areas where agro-ecological conditions did not allow any of the above changes and where the proximity of main roads, industrial zones or main cities (Ayutthaya, Bangkok) have both generated other labour opportunities and provoked a high level of land ownership transfer to speculators and urban-based buyers.

 It appears as a main evidence that an increasing differentiation of farming systems has occurred in the area during the last ten years, while sub-regions were preferentially evolving towards one or some of the above paths. In addition, in the last three years several factors contributed to sharpening the situation : TVs rice cropping suffered high levels of crop failure in 1995/96 and 1996/97 because of flooding and also in 1997/98 because of hectic rainfall during the crop establishment phase. This situation prompted RID to deliver exceptionally high supplies of water during the following dry-season (provided as a compensation). In addition, this happened to be concomitant with a surge in rice prices and triggered a crave for dry-season cropping, paving the way for a record area of 100,000 rai of triple-cropping in 1998.

These conditions - good water and price - provided farmers in the study area the incentive that was missing to engage in land development and embrace DS cropping, many of them for the first time. The responsiveness of TVs growers can be considered relatively high if one remembers that no assurance was given on whether such supplies could be renewed in the future.

 The dry season cropping boom provided an incentive for land development, which, in turn, is making the possibility to shift from TVs to HYV in the rainy season more attractive, by removing one of the main constraints. More generally, WS and DS rice cropping appeared significantly interlinked, not only in terms of calendar or techniques (DS cropping implies the use of wet broadcasting in the rainy season, even for floating rice), but also in the long term farmers' strategy.

Another highly significant event of the last ten years was the economic crisis in 1997, which put a brutal end to land buying and to speculation, slowing the worrisome trend of agriculture disappearance and injecting increased labour in the agricultural sector.

The future of the flood prone area of the Chao Phraya area is likely to be governed by a few factors : crucial will be the rate of double-cropping which will be allowed by the available water (possible tapping of additional resources, better management of the existent ones, "reduction" of the flooded area in some boxes, improved cropping techniques, etc). National policies and the economic environment will also contribute to set key parameters : price of rice, daily wage differential between urban and rural areas, labour opportunities in other sectors, land market, etc.