4. But now at the beginning of the 21st century the situation is quite different:

In many semiarid regions of the world population growth is putting pressure on water supplies for drinking water, for animals and for agriculture.
Agriculture and water projects based on high energy input and sophisticated technology appear more and more unsustainable.
At the same time, old re-approved or new technologies and / or modern materials made it possible to have a new approach to the construction of storage tanks and catchment areas.
All this led to a new expansion of rainwater harvesting systems both in regions where they had been used as well as in areas where they have been unknown until now.

Once again some examples to illustrate this point:

In the loess plateau in the North and Northwest of China where rainfall is low and groundwater is very scarce people have been experimenting a lot with rainwater harvesting. Agriculture in this region relies mainly on rain as water source. In the last years, the local government of the Gansu Province implemented a so-called "121" rainwater catchment project: the government supported each family to build one (1) water collection field, two (2) storage tanks and one (1) land to plant cash crop. The project has successfully settled the drinking problem for 1.3 million people (260,000 families) and their 1.18 million livestock. Since 1997, a rainwater catchment and irrigation project has followed up, aiming to supply water for supplemental irrigation with a high efficient water saving method. The rainwater is collected on the courtyard (Fig. 4) or slope field lined with concrete slabs and stored in the underground tanks. It is easy to create a water pressure for pipe or drip irrigation in this region. Cash crops such as vegetables, medical herbs, flowers and fruit trees as well as nurseries were planted. Farmers in the northern mountainous area of Yuzhong county have been very delighted about the vegetables grown in their own greenhouses, irrigated by rainwater tanks. This is the first time in history that greenhouses are built to grow such vegetables as pepper, eggplant, tomato and squash at an elevation of 2,300 m above sea level and with only 300 mm of annual precipitation. Rainwater harvesting has become a strategic measure for the social and economic development in this semiarid region.
In the semiarid region of Brazil, agriculture has been introduced only in the recent past. The local population had no possibility to experience with rainwater harvesting methods much less a possibility to learn how to live and work in a semiarid climate. But now, especially due to population growth and environmental degradation, people have to learn how to live in this rural semiarid region extending over 900 000 km². Reliable surface water is supplied for a very small area by the San Francisco River and the groundwater in the mostly crystalline subsoil is scarce and saline. Therefore rainwater is the most reliable source of water for humans and animals.
From experiences of the past and from other semiarid regions we learned that the sustainability of water harvesting systems is based on the combination of the basic needs of the farmers, the local natural conditions and the prevailing economic and political conditions of the region.
The people learn to live in a semiarid region by creating a new culture of relation to the environment and to water. This new relationship to the environment and water has been particularly fostered by the many grassroots organizations in the region. Nevertheless, there are still big irrigation projects underway along the Sao Francisco River. Large companies are planning to irrigate large areas to raise cash crops for export.

Three main topics were developed geared at guaranteeing peoples' sustainable living in Brazil's Northeast:

- Climate and water management: How does the semiarid climate work and what consequences does it have for agriculture. Establish preventive measures such as rainwater harvesting in order to have water reserves for the dry periods.
- Animal husbandry: Provisions for keeping small animals, especially sheep and goats, adapted to the semiarid climate. Storing of water and fodder for the dry months, de-worming, etc.
- Water harvesting agriculture: catch run off to have crops even in years with poor rainfalls; planting drought resistant crops like sorghum; planting trees like the natural umbuzeiro (Spondias tuberosa) well suited for a semiarid climate.

Together with the rural people, the water problem has to be managed in three ways, using all the available kinds of water supply (ground, surface and rainwater). It is necessary to have:

a - drinking water for every family (supplied by cisterns, shallow wells, etc.);
b - community water for washing, bathing and for animals (supplied by ponds, ground catchment rock-cisterns, riverbed-cisterns, shallow wells, etc.);
c - water for agriculture (supplied by sub-surface impoundments, supplemental irrigation, road catchments for irrigation of fruit trees, use of furrows for storing rainwater in situ = inter-row water harvesting);
d - emergency water for drought years (supplied by deep wells and smaller dams strategically distributed). This point is a transitory solution as long as points 1 - 3 are not completely achieved.

Different types of rainwater harvesting in Northeastern Brazil are:

Until now, among various cistern types used to resolve the drinking water problem in rual areas in Northeast Brazil, the concrete plate cistern made of cement plates (50 cm wide, 60 cm long and 3 cm thick) 14-gauge binding wire and plastered in and outside has been the most constructed cistern. As the adherence between the concrete plates sometimes is insufficient, tensions can cause cracks through which the water can leak.
For this reason, in the future the wire mesh concrete cistern (using a cast during the first construction phase) will probably be the most used and appropriate type for cistern construction in the region. This type of cistern hardly leaks and if so, it can be easily fixed. It is also useful for small and big cistern building programs (Fig.5).
An underground lime and bricks cistern remembers the Abanbars of Iran and the Chultuns of Mexico (Fig. 6).
In different parts of the semiarid region there is a revival of hand-dug rock cisterns; a traditional way to harvest water for the dry season. Their water is normally used for animals, but after filtering can be used for drinking purposes as well (Fig.7).
Sub-surface impoundments / dams store surface rainwater runoff for a later application: the barriers are dug below the ground surface in a shallow soil toward the impervious crystalline subsoil. Then earth or rockfill dams are built with a PVC sheet on the upstream face avoiding seepage. On the watered upstream soil it is possible to plant crops or fruit trees. In addition there is almost always an underground cistern to use the water for humans, animals or irrigation. Even in the first months of the dry season, it is possible to plant a second time and even in the driest years these impoundments are never without water (Fig.8).
The so called life-saving / supplemental irrigation impoundments collect rainwater runoff from a big natural ground catchment area. Downstream the impoundment people plant annual crops like beans, corn or sorghum. If there is a drought spill during the rainy season, people can water the fields with the help of gravitation from the impoundment. If they don't need the water, they can plant again in the dry season and use the water for irrigation of a second crop (Fig.9).
Inter-row harvesting in situ e.g. pre- and post seeding furrowing, partial ploughing or water stopping furrows. "In situ" rainwater catchments are suitable for the existing planting systems and they can be implemented with the help of machines or animals (Fig.10).