Rainwater utilization: A key measure to improve water ...

Rainwater utilization: A key measure to improve water resource management

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Paper presented at the Symposium on Low Cost Technology Options for Water Supply and Sanitation in Panglao Island, Bohol on October 12-14th, 2004 Johannes G. Paul, Ph.D. German Development Service c/o City Planning and Development Office Bais City, Negros Oriental, Philippines

1. Introduction Rainwater, before hitting a building or the earth surface, is a pure water source. And rainwater utilization is a practice as old as human society. Rainwater harvesting was applied for centuries and in many countries. And it was only in the 19th century that groundwater reservoirs could be tapped on a larger scale following the development of drilling and pumping techniques and the invention of electrical and fuel-driven engines. Consequently, the exploitation of groundwater resources is now the favored target for fresh water supply. However, the nature of tapped groundwater resources is often not known in the needed detail and in many cases water exploitation does not consider resource stability and extraction limits based on the local groundwater recharge rates. Development relies heavily on the availability of fresh water resources. At the present we witness a continuous growth of the global population. Hundred years ago 1 Billion people lived on earth. Now the global population counts more than 6 Billion people and more human beings live on earth than ever lived before. At the same time the water demand increases due to urbanization as well as agricultural and industrial development. But in many regions rainwater is still neglected as a fresh water resource and even regarded as waste and disposed into drainage systems or rivers instead of being utilized. The presence of water is essential for all forms of life on earth. The continents receive and store fresh water by interaction of climatic, topographic and geological conditions. The main driving forces of the natural water cycle are the sun and gravity. Basically, the water supply for terrestrial ecosystems

Symposium on Low Cost Technology Options for Water Supply and Sanitation, Bohol, Philippines, October 2004


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depends on rainfall or other precipitations and their storage in surface and subsurface systems. Rainwater is a renewable resource and provides recharge for surface and groundwater reservoirs. Globalization, industrialization and population growth changed the natural environments and land use patterns during the last hundred years in many ways. More and more land is used for settlements, industry and infrastructures. Forests were “clear-cut” and transformed into agricultural land. The results are soil erosion and increasing floods, but also decreasing surface and groundwater recharge. At the same time more water is needed due to population growth, industrialization and urbanization, which results in increasing pressure on other natural resources. More than half of the water used for drinking, washing and irrigating crops comes from groundwater resources (UNICEF, 2001). It is generally taken for granted that the groundwater drawn from wells is omnipresent and always available as well as clean and safe to drink. However, many cases are known, were groundwater utilization needed to be terminated as a result of overexploitation, decreasing recharge, salt water intrusion, contamination or poor sanitation. Compared to other countries, the rainfall patterns in the Philippines are favorable and would allow seasonal or year-round utilization of this water resource almost everywhere in the country. But there are many communities where the public water supply cannot cope the demand. And many opportunities for development cannot be realized or are delayed because of insufficient water supply.

2. Fresh water availability and water demand The earth surface receives roughly 500,000 km 3 precipitations/year. From that approximately 110,000 km3 are received by the continents, which cover 21 percent of the earth surface (LAMOUR & IGBOSIONY, 2001). Approximately 38,800 km3 from the various precipitations are directly transported back to the atmosphere by evaporation. The potential for Rainwater Harvesting is assessed with 14,000 – 25,000 km3 that represents a water volume 4 - 5 times higher as the available water from rivers on earth. The latter exceeds the volume of all available groundwater resources.


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Assuming an average water demand of 100 liter/capita/day the global rainfall would theoretically be sufficient to supply the domestic water demand of more than 30 Billion people (PAUL, 2003). However, rainwater is not evenly distributed over the various climatic zones and the different seasons of the year and the global water demand for agricultural and industrial activities is 8-9 times higher than the domestic water demand (RIJSBERMAN,2001). Following the population growth, the annual water consumption increased globally from 1,000 km3 in the year 1940 to now 3,500 km3. For the year 2025 more than 5,000 km3 potable water will be needed to supply the global population (KOENIG, 2001). The following graph gives an overview of the total water consumption for the various continents.

Worldwide Water Consumption from 1940-2025 in km3 per year

6000

World

5000

Europe

4000

North America

3000

South America

2000

Africa

1000

Asia

0

4 19

0

6 19

0

8 19

0

0 20

0

2 20

0

Australia + Ozeania

source: source: K.W. K.W. Koenig, Koenig, 2001 2001

It is alarming that more than 60 percent or 3,000 km 3 of the future global fresh water supply needs to be provided for Asia only 20 years from now. With the growing water demand and decreasing groundwater reservoirs, potable water supply is on the way to become a booming business. With a trend to encourage investment from private entities to support public functions, it can be expected that the private sector gets more involved in water supply. However, growing privatization may mean, that access to water becomes a function of economic markets and the water supply for the poor gets even more restricted.


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This scenario corresponds with increasing water prices, whereas further water stress may generate due to water rehabilitation measures, need for water treatment or water imports. Water import as substitute for insufficient public water supply is already accepted, for example as expensive water bottle supply. If users are required to pay the true cost of water, the biggest bills would be paid by industrial and agricultural consumers. Irrigation accounts for approximately 74 percent of the water consumption in the world, and many agribusinesses currently tap natural water resources for free or are the beneficiaries of publicly pumped groundwater at subsidized prices (UNICEF, 2001). Considering the population growth in developing countries, it is estimated that by the year 2025 about 17 percent more water will be needed to grow sufficient food and reduce hunger. At the same time, the worlds ecological systems are already under pressure and it is argued that no additional water can be withdrawn from nature without seriously damaging the resource base for future generations. It is estimated that a portion of 20 – 50 percent of the natural available water resources needs to be reserved to maintain a natural environment in the most river basins (IWMI, 2003). Rijbersman (IWMI, 2001) summarized the global situation of water demand and distribution for industrialized and developing countries as follows: Competing water uses – water distribution Water distribution between sectors agricultural

domestic

industrial

World

74 %

8%

18 %

Developing countries

85 %

6%

9%

Developed countries

42 %

14 %

44 %

source: F. Rijsberman, IWMI, 2001

Groundwater is the main source to satisfy the global water demand at the present. But groundwater is also the source of base flows for surface waters and wetlands. It is an effective buffer against drought. As global warming is expected to alter recharge patterns, the buffering function becomes more important.



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3. Rainwater – treated as liquid waste? Assuming an average daily water demand of 100 liter/capita/day, the Philippines have to provide 2,9 Billion m3 of fresh water every year only for the domestic sector. Presently, this water is mostly extracted from groundwater reservoirs. The cost equivalent would be approximately 87 Billion PhP-Peso/year, based on a water price of 30 Peso/ 1,000 liter. Consequently, a substitution rate of the public water supply with 50 percent rainwater would not only be very advantageous considering economic effects but could help to secure surface and groundwater resources and options for further development. Studying the actual water supply situation or the local rainwater management in the Philippines, it becomes obvious that rainfall is mostly not seen as water resource but often regarded as a hindrance for construction and project operation. Sealed areas, roofs of buildings and roads are provided with collection systems for the run off, but the latter is regarded as “useless” and only drained into the next river. Being a valuable fresh water resource before hitting the catchment area, rainfall becomes a waste when discharged into the drainage system. So far, there is no regulative system to manage rainwater before area sealing takes places in the Philippines. Construction permits for the domestic sector are given without questioning the volume, treatment and disposal of rainwater. In the Philippines there is also no “user fee” for using public drainage systems for rainwater drainage or other liquid waste disposals, whereas the budget to construct and maintain drainage systems is provided by the Local Government Units or other governmental agencies.

4. Rainwater and drainage assessment for the LGU Bais City To assess the options and benefits of rainwater harvesting, the Local Government Unit Bais City is used as an example. Bais City is located at the eastern side of Negros Island, 45 km north of Dumaguete City, which is the provincial capital of Negros Oriental. In the year 2000, the local Water District provided a total of 617,000 m3 fresh water for 2,500 households, which means an average water supply of 125 liter/capita/day. However, for the whole of Bais City 13,300 households are recorded, which means a central water supply service for only 19 percent of the inhabitants. The local rainfall in Bais is approximately 1,200 mm/m 2. Assuming an average roof area of 70 m2 per household and a potential to harvest 80 percent of the rainfall, theoretically 894,000 m3 rainwater could be harvested from the 13,300 houses in the Bais District. For the City Center and adjacent barangays a total of 220,000 m3 could theoretically be harvested from 3,300 houses.



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This calculation does not include other potential surfaces for rainwater harvesting such as commercial or institutional buildings (schools, colleges, churches, markets, city buildings etc.). The comparison between the produced fresh water of 617,000 m 3 by the local Water District and the potential for rainwater harvesting of 894,000 m3 indicates that rainwater collection could easily substitute more than 50 percent of the less sensitive water demands such as for laundry, house cleaning, gardening and sanitation in the city center, whereas the total rainfall collected from roofs of private houses provides more fresh water than produced by the local Water District. However, there are more sealed areas in Bais City. According to the Bais City land use plan from 1996, 1,994 hectares are classified as highly sealed areas. Assuming that 60 percent of the rainfall received by these areas is collected and drained, a total discharge of almost 14 Million m3/year results respectively 39,300 m3 run off/day. The main rivers in Bais City discharge in average 160,000 m 3/day (SA-A and BUSTILLO, 1997). If the proposed development plans of Bais City would be realized, such as additional housing projects, airport and road constructions, further 7,000 m3 average daily run off would result (PAUL, 2002). This portion may increase during times of heavy rainfalls when the hold back capacity, adsorption and evaporation rates on sealed areas are reduced. To summarize: The discharge generated by sealed surfaces provides a much higher run off than commonly believed and may well be responsible for increasing and more severe flood events !

5. Benefits of rainwater utilization Collecting the rain that falls on a roof or a sealed surface is a simple concept. Since this water is harvested independently of any central system, it promotes self-sufficiency and gives appreciation of this essential and precious resource. Collecting rainwater is not only water conserving, it is also energy conserving since the operation of a centralized water supply requires high energy input for pumping and distribution. A decentralized rainwater supply can use gravity flow if designed in the proper way. Rainwater harvesting also lessens local erosion and flooding caused by drainage from constructions, road development, land use changes, area sealing etc. The rain is instead captured and stored and only gradually released at a later time. Thus, storm-water run off, the normal consequence of rainfall, becomes captured rainfall and is available for a number of productive uses.



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Rainwater utilization offers many advantages:         

additional water for agricultural, industrial, commercial and private use, independent water supply, low cost water supply if gravity flow is applied, regulated drainage and river input, reduced soil erosion and transfer, additional water for public use and emergency response, reduced flood risk on roads and sealed surfaces, improvement of the liquid waste management system, improvement of river management.

Rainwater, before hitting a building or the earth surface, is one of the purest sources of water. Considering the hydrochemical quality, rainwater often exceeds that of ground or surface waters, because rain does not come in contact with soils or rock formations, where minerals and salts are dissolved. However, rainwater harvesting needs to consider and treat eventual microbiological and chemical components, which may be dissolved and collected from the catchment areas. The purity of rainwater makes it also an attractive water source for certain industries, for which pure water is a basic requirement.

6.

Environmental aspects of rainwater utilization

Rainwater collection is not only advisable to support the public water supply. Rainwater drainage systems are a basic component of most construction projects to handle storm water run off and to prevent flooding. Before area sealing, rainwater was available at this location for plant consumption, soil life and groundwater recharge. After surface sealing the rainwater needs to be drained, because it becomes a nuisance. In fast-developing areas, especially large cities, surface sealing will increase the run off and may finally create a new problem. Not only the drainage systems, but also the connected rivers cannot cope with the increasing effluent. Consequently, expensive riverbed and dam constructions have to be installed to prevent flooding resulting in ecological changes and hence decreased selfcleaning potential of the natural aquatic systems.

Deforestation of hinterlands contributes intensively to flooding depending on the size of a catchment area and the amount of rainfall, because the run off


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coefficient of open soils is much higher than that of forested lands. Agricultural areas cannot hold back as much rainwater and run off is higher as in forests (GOULD & NISSEN-PETERSEN, 1999). The following sketch demonstrates how area sealing and deforestation may change the level of a groundwater table and increase run off and evaporation due to land use changes. Consequences of Area Sealing

evaporation increasing

drainage + „flood risk“ increasing

Drainage and soil transfer increasing

groundwater recharge decreasing

Before .....

After area sealing

If rainwater is quickly drained from sealed sites or roofs, the extremes of storm water run off are significantly amplified. On the other hand, if rainwater is hold back efficiently this effect can be balanced. This also provides additional fresh water for various uses, which contribute to save groundwater resources. If the run off can be reduced, drought effects during the dry seasons can be lessened, because stored water is only released later and gradually over a longer period. For agricultural areas many methods are available to reduce run off and soil erosion and to increase the hold-back-capacity of water (ANSCHüTZ et al., 1997; IWMI, 2003). This can be realized with small scale constructions along contour lines such as terraces, ridges, dams, depressions and hedges, which help to stabilize soil, e. g. as a result of improved root growth, or contour plough and planting in moulds (DUVESKOG et al., 2003; YAHOLNITSKY, 2003). To balance the effects of land use changes, deforestation and surface sealing, rainwater needs to be hold back at the area of precipitation to allow a



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later domestic, agricultural or industrial utilization or to be used for surface or groundwater recharge. The following picture shows a seeping shaft, where drained rainwater could be stored for irrigation or groundwater recharge. Irrigation or groundwater recharge through subsurface constructions

or irrigation

Groundwater recharge

Alternatively, a subsurface gravel bed or mould can also be constructed to avoid quick drainage and loss of freshwater. This could also be realized with an artificial surface or subsurface water basin.



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7. Rainwater utilization for water supply The future water demand in the Philippines will depend on the further population growth, land use development, urbanization, industrialization and agricultural production. The pressure on surface and groundwater reservoirs may be amplified by resource overexploitation and increasing pollution of surface and groundwater reservoirs. Further hindrances are related to leaking water supply and waste water drainage systems and increasing application of synthetic substances in industrial and agricultural production. However, the degradation of watersheds and rivers lessen the natural self-cleaning potential. Additionally, inadequate solid waste management, the vast application of chemical fertilizers and chemicals for pest control threat the quality of natural water resources. Rainwater utilization can be regarded as one of the key-options to provide additional fresh water for the future. The present utilization of suitable surface and groundwater resources is often on its limits and further extension of exploitation could lead to resource depletion. The quality of surface and groundwater resources cannot be sustained in many regions, because of ongoing industrial and urban development. The development of new sources for fresh water supply on the other hand appears increasingly more complicated. In many watersheds other, competing claims for land use limit the protection and utilization of surface and groundwater resources. If the latter are on its limits, rainfall may be the only locally available water resource left for fresh water supply ! Presently, the water resource appreciation does not integrate rainwater utilization in the needed volume. Groundwater reservoirs are limited and cannot provide sufficient fresh water to satisfy the future water demand in many regions. The present water utilization scheme and the related resource appreciation is characterized with the following picture.



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Rainwater can be harvested at the points of consumption. Rainwater is a renewable resource and its utilization is linked with many environmental benefits. The set up of rainwater catchment systems can be designed in various scales and systems can be adapted to almost any requirement. Rainwater harvesting is in general a comparable simple technology of water supply and can be installed as decentralized water supply system. The future water supply should be seen as an integrated water supply tapping all available, suitable natural water resource outbalanced for the local water cycle. Because of the uneven distribution of water resources with rainwater as the largest water resource component, rainwater utilization should be given priority before using surface or groundwater reservoirs whenever possible. The future water resource appreciation is proposed with the following sketch.



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At the present, rainwater is mostly not considered as an important water resource. However, rainwater is the only renewable fresh water resource supplied by solar energy and gravity force, which the earth receives every year. All other existing surface and groundwater reservoirs are secondary water resources and depend on the recharge from rainfall or other precipitations. Even the polar icecaps will diminish or finally cease to exist, if there is no recharge from precipitations. Rainwater is abundant in the Philippines, but often not utilized or seen as a water supply alternative. Often, rainwater is treated as liquid waste and disposed respectively drained into the next river and from there into the sea where it is “lost” as fresh water resource.



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REFERENCES Anschütz, J., Kome, A., Nederlof, M., de Neef, R. And van de Ven, T. (1997): Waterharvesting and soil moisture retention. – AGRODOK-Series No. 13, 92 pages, Wageningen, The Netherlands Calder, J.R. (1999): The Blue Revolution – Land use and Integrated Water Resources Management. – Earthscan Publications Ltd., 1. Edition, London, United Kingdom, 192 pages Duveskog, D.; Mburu, C. and Forsman, A. (2003): Farmer innovations in water harvesting. – LEISA, Magazine on Low External Input and Sustainable Agriculture, June 2003, Vol. 19, pages 22 and 23 Gould,J. and Nissen-Petersen, E. (1999): Rainwater Catchment Systems for domestic supply. – Intermediate Technology Publications, London, United Kingdom, 335 pages IWMI (July 2003): Research update. – News of the Progress and Impact of Research of the International Water Management Institute, Sri Lanka, 8 pages Koenig, K. W. (2001): The rainwater technology handbook. – 1. Edition, Verlag Wilo GmbH, Germany, 143 pages Koenig, K. W. (2003): Gearing up for future water challenges with precast concrete parts. – BFT, Vol. 5, 2003, Germany, pages 52 - 58 143 pages Lamour, M.Y. & Igbojiony, D: (2001): Rooftop Rainwater Harvesting Systems in the Tropics: Two case studies. – Posterpresentation at the 10th International Rainwater Catchment Conference in Mannheim, Germany, September 10-14th, 2001 Paul, J. G. (2001a): Quantity and Quality Aspects in Water Resource Management. - paper presented at the PWMC-National Convention May 24/25th, 2001 in Ditucalan, Philippines for the Philippine Watershed Management Coalition, 20 pages Paul, J. G. (2001b): Rainwater utilization – Technical aspects, options and benefits for the water resource management and the environment. - Paper presented at the 1st Philippine Conference on Ecological Rainwater Harvesting, Iloilo City, Panay, November 3-5th, 2001, Philippines, 32 pages



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Paul, J. G. (2002): Characterization of solid and liquid waste sources and options for environmental improvement in Bais City, Negros Oriental. – Ph.D.-Thesis, Washington International University, USA, 204 pages Paul, J. G. (2003a): Rainwater. – Watershed Magazine, Vol. 1, No.1, published by the Philippine Watershed Management Coalition, Manila, Philippines, pages 19-21 Paul, J. G. (2003b): Rainwater – wasted or utilized ? – Watershed Magazine, Vol. 1, No.4, published by the Philippine Watershed Management Coalition, Manila, Philippines, pages 6,7 Perez, C. (1991): The Philippines, recommends for watershed management. – Los Banos, Laguna, Philippines, 88 pages Rijsberman, F. (2001): Balancing Water Uses: Water for Food and Water for Nature. – Newsletter of The International Conference on Freshwater, Bonn, Germany, Issue 2, page 3 Sa-a, P. and Bustillo, R. A. (1997): Water quality of Bais Bay. – Silliman Journal, 37, (3-4), Dumaguete City, Philippines, pages 75 - 92 UNICEF (2001): Groundwater: the invisible and endangered resource. – fbr-wasserspiegel 4/01, 6. Jahrgang, page 21, Germany, June, page 175 Wolf, A.T. (2001): Transboundary Waters: Sharing Benefits, Lessons Learned. – Newsletter of the International Conference on Freshwater, Bonn, Germany, Issue 2, page 3 Yaholnitsky, I. (2003): Holding the Rain. – LEISA, Magazine on Low External Input and Sustainable Agriculture, June 2003, Vol. 19, pages 24 and 2
