Reservior & Farm Pond Design

Lesson 29 Water Quality of Harvested Water and Environmental Considerations

29.2 Water Harvested for Human Consumption

Water for drinking purposes and other domestic uses must meet certain qualitative standards. World Health Organization (WHO) has prepared guidelines in 2004 for the provision of safe drinking water, including quality standards and information on the roles and responsibilities of various stakeholders involved in providing drinking water. However, the practical application of these requirements varies from place to place depending on the living standard of the community and type of water source.

The main water quality indicators of drinking water are characterized by their physical, chemical and biological parameters. The list of the main indicators/parameters includes:

• Alkalinity

• Colour of water

• pH

• Taste and odour

• Dissolved salts (sodium, chloride, potassium, calcium, manganese, magnesium)

• microorganisms such as fecal coliform bacteria (Escherichiacoli), Cryptosporidium, and Giardia lamblia

• Dissolved metals and metalloids (lead, mercury, arsenic)

• Dissolved organics such as dissolved organic matter dissolved organic carbon etc.

• Heavy metals

Thus, the basic requirements for safe drinking water can be outlined as:

• Free from disease causing organisms

• Free from compounds that have an adverse effect  on human health

• Fairly clear(low turbidity and little colour)

• Without offensive taste or smell

29.3 Water Harvested for Crop Production

The quality of water used for irrigation is an important factor in productivity and sustainability of crop production. In evaluation of irrigation water, emphasis is laid down upon the physical and chemical characteristics of water and only rarely on any other factor considered important. In most of the locations where water harvesting for agriculture is practiced, the physical properties of water are much more important than chemical properties. In this context, attention is focused on the quantities of solids, nutrients and in rare cases the pollutants transported by the particles and the points of their deposition. Water infiltration is hampered when the sediment is rich in clay and/or contains relatively high sodium or low calcium content. If too little water infiltrates into the soil that means when more water evaporates, the crop may suffer from moisture stress and likely to wilt when the situation lingers. 

Some of the chemicals used to treat the catchment and their final products may be harmful for the crops and affect plant growth. Also runoff water runs over salt bearing rocks carries dissolved salt that reduces crop growth. The parameters that influence the irrigation water quality are given in Table 29.1.

Table 29.1. Irrigation Water Quality Parameters

(Source: http://www.fao.org/docrep/003/t0234e/T0234E01.htm#ch1)

Water parameter	Symbol	Unit	Usual range in Irrigation Water
(i) Electrical Conductivity   (ii)Total Dissolved Solids	ECw   TDS	dS/m   mg/l	0 – 3   0 – 2000
Calcium	Ca++	meq/l	0 – 20
Magnesium	Mg++	meq/l	0 – 05
Sodium	Na+	meq/l	0 – 40
Carbonate	CO--3	meq/l	0 – 0.1
Bicarbonate	HCO3-	meq/l	0 – 10
Chloride	Cl-	meq/l	0 – 30
Sulphate	SO4--	meq/l	0 – 20
Nutrients
Nitrate-Nitrogen	NO3-N	mg/l	0 – 10
Ammonium-Nitrogen	NH4-N	mg/l	0 – 05
Phosphate-Phosphorus	PO4-P	mg/l	0 – 02
Potassium	K+	mg/l	0 – 02
Miscellaneous
Boron	B	mg/l	0 – 02
Acid/Base	pH		6 – 8.5
Sodium Adsorption Ratio	SAR	(meq/l)0.5	0 – 15

29.4 Water Quality Considerations Related to Water Harvesting Systems

Various water harvesting systems for storing surface runoff and flood water are designed to meet the irrigation requirement of crops. Such systems may collect water from micro-catchments, long slopes and floods during rainy season.

29.4.1 Runoff Water from On-Farm Micro-Catchment Systems

Runoff water from the catchment is used directly without interim storage, to irrigate crops. The nutrients present in such runoff water are beneficial, but the other substances present in the sediments may be harmful as they reduce the physical quality of the water. When the catchment areas are treated with chemicals like herbicides and pesticides or even with chemical fertilizers before rainfall events, the harvested water is likely to get contaminated and pose a threat to the crops.

29.4.2 Long-slope Water Harvesting

Runoff water from long slopes is used to provide additional water to trees, bushes and annual crops on the cropped area. In most cases, the water is conserved directly in the soil profile although it is sometimes stored in cisterns, ponds or reservoirs. This water may also be used for livestock requirement and other domestic purposes. The catchments are either left in a natural state or cleared of vegetation and stones. In latter case, there is a high risk of soil erosion and hence the probability of sediment transport to stream channels and into the storage bodies is more. For small storage facilities, it may be possible to construct a sediment trap upstream of the reservoir. Most of the suspended sediments settle at the bottom of the trap and the clear surface water is directed towards the main storage facility. Part of the runoff water from the sediment trap may be lost due to evaporation but, in return entry of clean water increases the life of the storage structure. The best strategy for dealing with sediments is to prevent water and wind erosion. Provision of fencing or hedges around the ponds or reservoirs keeps animals and people away from direct contact of water. Sometimes direct drinking of water by animals from surface ponds contaminates the water. 

29.4.3 Flood Water Harvesting

Flood water harvesting is commonly used to supply water to trees, bushes and annual crops. In a number of cases the water is stored in ponds and reservoirs. The water moves soil particles, which may carry nutrients as well as chemical pollutants. Sedimentation of distribution basins, canals and storage bodies is the consequent common problem because of large scale transportation of sediments with floodwater.

Deposition of sediments also changes the geometry of the section of ephemeral river beds. It reduces the channel cross section and also changes the hydraulic gradient. The effect is likely to spread along the upstream as well as the downstream of the section resulting in overflow at various sections along the bank during high flows. Banks may be eroded and the river bed may even change its course. During floods, large amount of sediment may be deposited on the floodplain, which may be beneficial in the long term but not desirable as far as the short term benefits are concerned. The more obvious problem produced by sediments is the loss of storage capacity of reservoirs. It is not uncommon for storage facilities to be completely filled with sediment within a few years of construction.

One efficient means to trap sediments is by constructing rock dikes across valleys to capture surface water. The sediments will settle and create a flat surface for growing crops, particularly trees with deep roots to reach the water stored in the trapped sediments. Stored water needs to be protected not only from evaporation and seepage loss but also from contamination. Contamination occurs mainly from human or other animal contact. Similarly stored water needs to be protected from disease vectors such as mosquitoes, files and mollusks.

29.5 Impacts on Downstream Ecosystems and Biodiversity

Usually aquatic flora and fauna develop in concert with existing water regimes. Implementing a water harvesting system alters the flow regime, which may cause some species to become stressed and die out. Other species that prefer the new flow regime will then colonize the riverbed and flood plain. These changes may be considered either desirable or undesirable based on their impact on the ecosystem. In most locations not only the aquatic species are adapted to the flow regime, but also the lifestyle of other species including humans is observed to be gradually fitting into the changing flow regime. In recent years it has been realized that many of the changes produced by significant modification flow regimes are undesirable. Downstream degradation of the aquatic flora and fauna produced by cutting off all flows will eventually be drastic and irreversible. Policies are being formulated to reduce the changes in the flow regimes such that the general environment downstream will not be drastically modified.

In most of the situations, the aquatic environments are not sufficiently understood for developing appropriate guidelines. Post-development flows are being set at about 25% of the pre-development situation with similar variability, but this is an arbitrary figure which has no scientific basis. Presumably it will be adequate to support some species, while others will disappear. Environmental flows, the flows needed to sustain the naturally occurring species, are difficult to define and are currently the subject of scientific and political debates. For many streams, maintenance of the flows necessary to support the natural systems means virtually no development. In other regimes particularly in semi-arid and arid regions, very little is known about possible effects of developing the very limited water resources. These effects may not be immediately apparent, but in the long run they will be noticed, and by then it would probably be impossible to reverse the effects even if there is a desire to do so.

However, water harvesting projects can also contribute to higher levels of biodiversity through demarcation of small areas at various locations (each of about 100 m²) for the development and multiplication of natural flora by erecting fence around, prohibiting animal grazing and supplying harvested water.

29.5.1 Water-borne Diseases

Any form of water resources development through water harvesting intervention causes changes in natural conditions. Many of these changes offer opportunities for multiplication of disease vectors with devastating effects.

In regions where malaria, dengue fever or similar insect-transmitted diseases are endemic, storage of water on the surface needs to be accompanied by precautionary measures to prevent the water becoming a breeding site for these disease vectors. Where schistosomiasis is prevalent, measures must be taken to control the snail that is the intermediate host of the parasite.

It is important that planners, decision-makers and financiers take health issues into consideration when planning for any water resource development project. This will often require changes to the scheme and may raise the cost of the project. But with innovative ideas, the changes and the extra costs should not be very large. Examples are to be taken from other irrigation projects in regions where river blindness and schistosomiasis are endemic.

29.5.2Thermal Stratification

Thermal stratification refers to temperature differences in the water at various depths particularly in ponds or large lakes of depth greater than 2 m. Energy from solar radiation heats the surface layers and the heat is transferred to the lower layers by mixing with water and wind effects. However, warm water is lighter than cool water and wind induced circulation cannot mix them with deeper layers of cool water. This causes thermal stratification. The layer relatively warm surface water is known as the epilimnion, and the bottom layer of cooler water is known as the hypolimnion. The temperature changes rapidly with increasing depth in the transition zone between the epilimnion and hypolinion and this transition zone is known as the thermocline (Fig. 29.1).

Thermal stratification influences the photosynthetic activity of plant species in the ponds and consequently the dissolved oxygen in the water. It also influences the chemical stratification in water bodies. For pisciculture purposes the water depth in the ponds is kept around 2 m and in such cases the changes due to thermal stratification are undesirable. It is necessary to understand the water quality aspects in ponds in relation to the use of the water.

Fig. 29.1.Thermal stratification in pond.

(Source: http://www.fao.org/docrep/field/003/ac174e/AC174E02.gif)

Keywords: Water quality, Environment, Ecosystem

References

• Murthy, V. V. N. and Jha, M. K. (2011). Land and Water Management Engineering. Kalyani Publishers.

• Owesis, T. Y., Prinz, D. and Hachum, A. Y. (2012). Rainwater Harvesting for Agriculture in the Dry Areas. CRC Press Publication.

• http://www.fao.org/docrep/003/t0234e/T0234E01.htm#ch1