Watersheds and Water Quality
If you were a drop of water, as soon as you hit a piece of land, how and to where you flow would determine what watershed you are in. Or more formally: Watershed is defined as the land area in which all of the incoming precipitation drains to the same location as a result of its topography. All land on Earth is a part of a watershed, whether small, big, narrow, or flat. While some may be closed ones and draining into a lake, most eventually drains to a major river, which discharges into the seas/oceans. One river’s drainage area can be compromised of hundreds of small sub-watersheds.
The watersheds provide us with the raw water source which eventually gets treated and distributed to our homes. Topography, soil thickness and type, land use&management practices, type of vegetation all have great consequences to the water quality and quantity we get downstream. Water that runs through a medium will take up certain pollutants and contribute to the pollution at the water supply source. The higher the pollutants in the water supply, the more treatment steps required to get good quality water running through our taps. It also affects the quantity of water, as we experience droughts or adverse effects of climate change, this becomes critical.
Based on the dominant use, we can classify the watersheds as forested, agricultural and urban. The major differences about these are the land use, how fast the water moves and how clean/dirty it is when it reaches its destination water body.
A forested watershed will be able to store more water in the soil and recharge the groundwater, whereas if the land is used for agriculture or urban development, runoff will cause not only the pollutants to enter the stream but also there will not be enough time for the water to be absorbed by the soil, therefore recharging the groundwater supply. Soil thickness and compactness are different with each watershed, especially between forested and urban. In urban areas, the soil is more compacted, allowing less path for air and water to circulate, and has less organic material.
The other major issue we see is the pollutant load into the receiving water bodies. Agriculture-oriented watersheds will see nutrient (nitrate and phosphorus) levels much higher than urban and forest watersheds. These pollutants cause algae growth in the water body, and will reduce the dissolved oxygen levels, causing the hypoxic (low-oxygen) zones. For example, in the US, the Mississippi River basin includes a lot of farmland, which contributes to the nutrient loading and the dead-zones in Gulf of Mexico. Casali’s paper cited a survey done in the forested and pasture watersheds which showed average nitrate release is 2.5 kg/ha-year and 3.5 kg/ha-year for forest and pasture watersheds respectively. Average phosphorus levels were recorded as 0.1 kg/ha-year for forested and 0.3 kg/ha-year for the pasture. It is cited that 0.03 mg/L phosphorus found in water puts that supply in eutrophication risk. That means algae and oxygen depletion, and risk to the vitality of the water body.
Erosion and sediment as pollutant are the other big issues regardless of the type of watershed. Erosion happens by the detachment of the soil particles and their transport. Once the transported particles deposit in a location, sedimentation occurs. Sedimentation rate and amount will change based on the water stream’s flow velocity and the shape/profile of the channel. Forested watersheds can provide buffer to heavy rainfalls by intercepting up to 25% of the rain, reducing runoff and erosion. However, logging activities and their proximity to the water source can also make a big difference in these numbers. In a study done in the Chesapeake Bay region, it was calculated that a decrease of 24% in forested area increased the runoff by 19%.
In Turkey, there are 26 river watersheds and five of them make up half of total water flowrate in Turkey. 70% of the watersheds discharge into the surrounding seas. Currently Turkey is categorized as a country experiencing water stress with 1519 m3/capita-year. Decreasing flowrates in the watersheds (16% between 1995 and 2002, Odemis and Evrendilek, 2007) and increasing population will eventually exacerbate the danger we are in already and will put us in closer to being “Water poor” with 1120 m3/year in 2030. The population density does not always correlate with the water availability in the watersheds in the region. For example, 28% of the population is in the Marmara region whereas the water flow provided by the watersheds in the same region corresponds to only 4% of the total flow. Due to the heavy industrialization and urbanization in the area and the lack of proper wastewater treatment, the watersheds in this region also has the highest pollution. Heavy metals, organic and nutrient loading are the major contributors to the pollution not only in Marmara Region but in the majority of the Turkish watersheds (Akin ). Odemis and Evrendilek also noted that there has been an increase in the pH and the temperature in substantial portion of watersheds they surveyed. Seeing elevated water temperature means lower dissolved oxygen available in these watersheds.
70% of the usable water provided by watersheds is consumed by the agriculture sector. The landuse in watersheds is also changing rapidly from forested to more pasture and urban type. Combining the thirst quenching with providing oxygen, forested watersheds are the ones we need the most, however, also the ones that is disappearing rapidly. They are losing to agriculture or urban developments. If no action is taken, per WWF, 230 million ha of forest will disappear by 2050. This is really disturbing considering this type of watershed not only provide clean water but store carbon, provide a healthy ecosystem for humans and wildlife, provide jobs in the industries which depend on forest. Based on a 2012 literature research, there are a 500 million forest-dependent people, and 200 million of this is indigenous people. In developing countries, about 1.2 billion people rely on agro-forestry farming systems.
Intertwined characteristic of the natural resources shows up itself here in watersheds. The problem with managing a watershed or implementing practices to prevent water pollution lies in the fact that watersheds do not define themselves with geo-political borders. The water flows through borders of different states, municipalities and countries, taking the pollutants with them. For example, Turkey has Meric-Ergene, Firat-Dicle, Coruh, Asi and Aras watersheds reaching beyond Turkey’s borders into our neighbors’, compromising 36% of Turkey’s national water potential. The management practices can be a source of great conflict if one government body’s project causes the quality and/or quantity deterioration in the neighboring community. However, watershed protection and management can and should be a great opportunity to reach out to different communities and stakeholders to work together. It is up to us to achieve clean, reliable water source for our livelihoods and our ecosystem.
References (Not Linked within the Post)
Edwards, P.J., Williard K.W.J and Schoonover J.E. “Fundamentals of Watershed Hydrology”, Journal of Contemporary Water Research and Education, April 2015
Casali J. et al, “Sediment Production and Water Quality of Watersheds with contrasting land use in Navarre (Spain), Agricultural Water Management , 2010 , Issue 97.
Akin M. and Akin G. “Suyun Onemi, Turkiye’de Su Potansiyeli, Su Havzalari ve Su Kirliligi” Ankara Universitesi Dil ve Tarih-Cografya Fakultesi Dergisi, 2007, Issue 47-2
Odemis B., and Evrendilek F., “Monitoring Water Quality and Quantity of National Watersheds in Turkey”, Environmental Monitoring and Assessment, 2007, Volume 133