The Pa Sak River, named after the majestic teak forests at its source, can be a vision of breathtaking beauty in the mountain streams and bountiful forests of its upper reaches. Its basin is a long, narrow strip of land flanked by mountain ranges. As the river flows south it is used for agriculture, domestic and industrial purposes, and offers popular sightseeing and recreational areas. From its source in Dan Sai district of Loei province (almost directly north of Bangkok) the Pa Sak River flows through Phetchabun, Lop Buri and Saraburi provinces, to join the Chao Phraya River in the province of Ayutthaya, where it is part of the water perimeter of the ancient capital.
Historically, the inability to manage, store and distribute the water of the river meant that those who lived beside it often faced difficulties. In the monsoon season, it often rises over its banks to inundate thousands of rai of productive agricultural lands. Extremes of drought and flooding due to rainfall patterns and the topography of the Pa Sak River Basin curtailed hopes for basic food security, leading to economic loss.
Dry season conditions meant that crops often failed in a parched landscape under a scorching sun. Farming was done with great uncertainty, risk and expense. Providing water for aquaculture such as fish, soft shelled turtles and frogs, and for poultry, cattle, pigs and kitchen gardens, was difficult and expensive. Monsoon floods threatened livestock with being swept away, while fish farms simply dispersed in the flood. Not only were cash crops and homes affected, but community assets such as temples, schools, community centres, markets and infrastructure - roads, railways, bridges - were also at risk. Such extremes disrupted the lives and destroyed the hopes and dreams of the inhabitants.
A major tributary east of the Chao Phraya River Basin, the Pa Sak River can be divided into four zones: the Upper Basin, the Phetchabun-section Basin, the Middle Basin and the Lower Basin.
The Upper Basin stretches from the river's source in the Phetchabun ridges of Loei province to Lom Sak district in Phetchabun province. High and steep mountains are characteristic features. The Phetchabun-section extends south from the Upper Basin to Muang, Nong Phai and Vichien Buri districts of Phetchabun province, through a landscape of alternating low hills and plains on both sides of the river used for orchards and rice cultivation. The Middle Basin extends from Vichien Buri and Srithep districts of Phetchabun, through Chaibadarn, Tha Luang and Pattana Nikhom districts of Lop Buri province, to Kaeng Khoi district in Saraburi province, and is characterized by hilly terrain. It provides the location of the Pa Sak Dam and its reservoir. The Lower Basin includes Kaeng Khoi, Muang and Sao Hai districts in Saraburi and Tha Reua, Nakhon Luang, and Muang districts of Ayutthaya. It features low-lying flood plains with fertile soil for agriculture and potential for irrigation development.
Heavy rainfall brought in by the south-west monsoon usually occurs from May through October. Annual rainfall averages 1250 mm/year, with a maximum of 255 mm in September. During the dry season, November through April, total rainfall amounts to only 150 mm, with only 8 mm in January.
Consequently, there is a vast difference between the amount of rainwater runoff during wet and dry periods. In the rainy season, the runoff can reach 2.2 billion m3, with a maximum of 920 m3 in October. During the dry season, average runoff amounts to some 200 million m3 per month with only 17 million m3 in March.
The total area of the Pa Sak River Basin is some 9 million rai (1.5 million hectares or 14 500 km2), of which 80 percent is used for agriculture.
| Mountains, forest, homes |  1.8 million rai 2880 km2 |
| Farms, pasture |  3.2 million rai 5120 km2 |
| Rice |  4.1 million rai 6560 km2 |
As in most of Thailand, rice is the primary food crop in the Pa Sak River Basin. Agriculture has changed radically in recent decades, altering ways of living, thinking and belief - and the experience of community. Rural life was once the basis of national culture, but as farm families changed from subsistence agriculture to market oriented agriculture, food production became a secondary concern. Drawn to flourishing industrial and urban service sectors and attracted by prospects of higher income, farmers left their land. However, with the recent economic downturn, farmers who had become city-dwellers were the first to experience unemployment and related social problems.
For the people of the Pa Sak River Basin, Thailand's rapid economic development in the past seemed to have negatively influenced the comfort and security of traditional community life. Gone was the community spirit of sharing surplus food and sharing labour. Surplus food came to be measured in monetary terms, to be marketed, not shared.
In 1960, agriculture represented some 40 percent of Thailand's Gross Domestic Product (GDP), and two-thirds (64 percent) of all employment. As industrial-technical investment increased, agriculture's proportion of GDP dropped dramatically to 11 percent in the mid-1990s, even though total land area devoted to agriculture was only marginally reduced and agricultural employment declined minimally to just 60 percent.
Before the dam was constructed, the project area was mainly rain-fed farmland, orchards and pasture. Rice and maize were normally cultivated only once a year, during the monsoon season, but crops were often half the estimated potential due to either flood or drought.
As can be seen in the table below, the second rice crop grown in the off season (dry season) is comparatively negligible due to the lack of supplementary irrigation water compared with the area cropped in the monsoon season. Due to the dam and the new irrigation schemes, 131 300 rai can in future be cultivated in the off season as well.
Unpredictable flood and drought conditions combined made agriculture a very risky business. Comparing data for “first” or “major” rice planted in comparison with the area actually harvested in Lop Buri and Saraburi provinces for the 1992/93 season, for example, indicates the erratic nature of production and yield. In 1992/93, Lop Buri farmers planted 948 438 rai of rice, but harvested only 593 120 rai - a loss of 355 318 rai or 37 percent of the projected crop. Losses in Saraburi province were similar, but less dramatic. In the same year 416 174 rai were planted but only 398 445 rai was harvested - a loss of 17 729 rai or 4 percent of the projected crop.
Such losses represent lost agricultural investment, such as seed, fertilizer and pesticide, as well as lost labour investment, and contributed to the trend of farmers leaving their lands to seek their fortunes in the city.
First (Major) and Second Rice Crop: Area, production, and yield by province 1992/93 and 1993
| Planted Area (rai) | Harvested Area (rai) | Production(tons) | Yield per rai (kg) | Province | ||||
| First | Second | First | Second | First | Second | First | Second | |
| 1992–93 | 1993 | 1992–93 | 1993 | 1992–93 | 1993 | 1992–93 | 1993 | |
| 948 438 | 776 | 593 120 | 776 | 273 150 | 426 | 461 | 549 | Lop Buri |
| 416 174 | 3383 | 398 445 | 3291 | 146 493 | 1539 | 368 | 468 | Saraburi |
