Abstract

Water shortage, flood control and pollution control have long been considered as the major hydrological problems resulting from urbanization. Comparatively, less attention is paid to the rehabilitation of the natural hydrological cycle in pre-urban area. In the viewpoint of ecology, one of the tremendous impacts of urbanization on the environment is the fatal destruction of the primary hydrological cycle in the area. So the rehabilitation of such cycle is of the most importance. Based on the results of a research project relating to rainwater harvest in urban area, a new approach is proposed in this paper, which could be used to gradually rehabilitate the primary hydrological cycle in built-up area, i.e. the hydrological cycle before a rural area is urbanized.

Key point is to regulate the proportion of the total volume of rainfall that appears as surface runoff with help of artificial hydrological connections produced on the basis of local conditions in climate, topography and hydrology. Key factors concerned with this type of hydrological connections are discussed in detail: space for, capacity of and water quality in the channels. One of the practical modes, for example, is the beneath-lawn channel that mainly consists of three parts: soil, filter and water storage space. The volume of latter depends on the local precipitation and the area of the catchments.

Research results show that the rehabilitation of the primary hydrological cycle in urban area is not only necessary but also feasible. A lot of parameters about the artificial channels should be calculated carefully before construction. As to the long-term effect of the rehabilitation on the local climate, further study is needed.

Key words: rehabilitation hydrological cycle urbanization

1. Introduction

Rapid urbanization and population growth have brought about dramatic changes in hydrological cycle. It is not overstated that water has become one of the issues concerned by all communities around the world. From the UN Millennium Summit’2000 to the Summit on Sustainable Development’2002 in Johannesburg, South Africa, water was and remains the focus of public’s attention. Everywhere, water use is increasing. More and more people are facing water shortage. More than 50% of urban populations do not have proper access to safe drinking water. In China, for example, about 300 out of the 666 cities are deficient in water supply. Among them 114 suffer the most serious water shortage. The average run-off per capita is only 2213 m³, less than ¼ of the world means. Water shortage in China is up to 30-40 billions m³ and the water shortage in cities, 6 billions m³. Water shortage, flood and water contamination are the main obstacles for the sustainable development [1]. Although a lot of solutions to the water issue have been proposed, including water saving, seawater desalting and rainwater harvesting and so on, water problem remains, and is getting worse somewhere or sometimes. Nowadays, humans already use fifty-four percent of all the fresh water in rivers, lakes and underground. There are some estimates that this rate will reach seventy percent by 2025. One of the reasons for this situation is that less measurements is taken to remove the root cause of water problem, namely the human-made pressure on ecosystems. According to the World Resources 2000-2001[2], such human-made pressure today is “far outstripping the natural processes of ecosystem change.” Water problem could only be solved when it is considered as an integral component of the ecosystem. In the viewpoint of ecology, urbanization results not only in the excessive withdrawals of groundwater, in the reduction of groundwater table, but also in the cut-off of the normal recharge of ground water. One of the tremendous impacts of urbanization on the ecosystem is the fatal destruction of the primary hydrological cycle in pre-urban areas. Although many factors might be responsible for the problems mentioned above, but the traditional geoethic concept about the human-nature relationship, "human is the master of nature", is the very reason ^[3]-^[4]. So major changes should be taken in the ways people use water and rehabilitation of hydrological cycle should be a task of top priority.

2. Human interventions in hydrological cycle

As well known, water is one of the vital factors for human and all living beings. Generally, water, unlike the fossil mineral resources, is renewable. It should be inexhaustible. Nevertheless, the renewability of water, especially the fresh water on which the human's life depends, is conditional[5]. Seasonable recharge of the groundwater through natural hydrological cycle is the very condition. A natural hydrological cycle in a pre-urban area should be like this: “Precipitation that falls on land is dispersed in several ways. The greater part is retained in the soil near where it falls and is ultimately returned to the atmosphere by evaporation and transpiration by plants. A portion of the water finds its way over and through the surface soil to stream channels, while other water penetrates farther into the ground to become part of the groundwater”[6].

As shown in Figure 1, main effects of urbanization on hydrological process are the increase of population density and building density. It is the urbanization that destroys this cycle. Land drainage, land sealing, and wet cuts are some of the typical human interventions in hydrological cycle[7][8]. Among them, different kinds of land sealing, like increasing buildings construction and traffic area, not only causes an increased and accelerated runoff, but also prevents groundwater recharge by preventing the slow infiltration of water, in particular during snow/frost thaw. Runoff water from sealed housing and traffic areas is normally unfiltered and contaminated with chemicals. To certain extent, land sealing could be regarded as the profound reason for water shortage, flood and water contamination in urban area. Unfortunately, this land sealing process is almost inevitable in urbanization. Any measurements to be taken for rehabilitation of hydrological cycle should have the capability to restore the hydrological cycle under the condition of land sealing. A new approach to rehabilitation of hydrological cycle will be discussed in the next section.

3. Nature-oriented modes for rehabilitation of hydrological cycle

With the process of the urbanization, impermeable materials, like concrete, asphalt cover more and more land surface in urban area, which cut off the natural hydrological connection between precipitation and groundwater. So space for nature water cycle in an urban area is getting less and less. Although some pieces of green land could be found in a city, they are normally human-oriented. Little attention has paid to the ecological functions of rainwater in the compensation to the hydrological cycle. To realize the rehabilitation of hydrological cycle in pre-urban area under land sealing condition, the destroyed natural hydrological connection between precipitation and groundwater must be reestablished at first. Key factors relative to the reestablishment of hydrological connection include space for, capacity of and water quality in the new connection.

Space for the new connection mentioned above mainly serves surface detentions and interception storage. Capacity of the new connection means its allocation capacity: regulation of the water volume for evaporation, runoff and infiltration. As to the water quality in the new connection, it is the determinant factor for recharge of groundwater. It is quite evident that reestablishment of such hydrological connection is of great importance for rehabilitation of hydrological cycle. To avoid the negative effects of human interventions upon hydrological cycle, the new connection should be nature-oriented. In other words, it should be nature-oriented: rainwater should not be discharged off as usual, but managed to recharge the groundwater as in a natural cycle.

Based on the research results of a project relating to rainwater harvest in urban area, a nature-oriented approach is proposed for rehabilitation of hydrological cycle. Kern technology of this approach lies in the regulation of the rainwater in urban area through the artificial hydrological connection between precipitation and groundwater.

Storage basin beneath lawns is one of the operational modes that have proved to be useful for the purpose mentioned above. This experimental basin covers an area of 50 m² with a filtration layer about 45 cm in thickness (Fig.2). Fig.3 shows the construction of such a basin. Continuous records of three years indicate that the surface runoff in the urban areas can effectively be cut down with the help of the infiltration materials. Even when the daily rainfall exceeds 100mm, no surface runoff appears in the experiment area. Comparison of the precipitation and water level changes in the basin beneath lawn clearly demonstrates that lag time of the surface runoff can effectively be extended to about 24 hours (Fig.4). In addition, stored water in the basin can not only serves gradual groundwater recharge, but also serves the allocation of the precipitation, e.g. for fountain spring, cleaning road surface, spraying flowers and grass etc. This mode is more appropriate for rainwater utility in residential areas.

Another operational mode is rainwater collection from rooftop. A detention basin is needed for this mode. Rainwater is organized and directed through pipes into the basin. An existing pond and a group of tanks are used for the water collection (See Fig. 5 and Fig. 6). In comparison with the storage basin beneath lawns, cost of the rainwater collection system in this mode is lower, but the quality of the collected water is lower, either. Further treatment is necessary before the allocation of the collected water.

Fig.2：Storage basin beneath lawn

Fig 3: Storage basin beneath lawns. Layers downwards: grass, soil, filtration mattress, fine gravels, gravels, support, basin.

4. Conclusions

Based on the discussion above, following conclusions could be drawn:

o Land sealing caused by urbanization process is one of the profound reasons for the destruction of natural hydrological cycle in pre-urban area.

o Rainwater regulation and rehabilitation of hydrological cycle in urban area is not only necessary but also feasible;

o Traditional concept about rainwater management in urban areas should be give up. Rainwater in urban area should not simply be discharged off, but collected in artificial way and reallocated in nature-oriented mode;

o Key factors relating to the reestablishment of hydrological connection include space for, capacity of and water quality in the new connection;

o Operational modes for rehabilitation of hydrological cycle include storage basin beneath lawns and rainwater collection from rooftop;

o Experiment data show that surface runoff in the urban areas can effectively be cut down and the lag time could be extended when suitable infiltration materials are selected.

o Further study about the long-term effect of the rehabilitation on the local climate is needed.