Lecture 7: Human influence Flashcards

Question

Land Cover/Land Use Change on the Water Cycle

Answer 1

• Land use impacts on the surface water balance and the partitioning of precipitation into ET, surface runoff and GW flow • Also changes in albedo (e.g. dark forest to light crops) alters the energy budget and therefore ET • Surface runoff and river discharge generally increase where natural vegetation (especially forests) is cleared • E.g. Tocantins River in Brazil showed a 25% increase in river discharge between 1960 and 1995 coincident with expanding agriculture

Answer 2

• The removal of trees (deforestation) has a major impact on the water cycle • Trees generate a great deal of ET, which is then recycled to precipitation further downwind. • Deforestation therefore contributes to changes in the water cycle in other regions. Percentage change in precipitation due to 2000–2050 business-as-usual deforestation of the Amazon basin. a) Wet season; b) dry season. (Spracklen et al 2012)

Answer 3

This map shows gridded fields of changes in river discharge (delta Q) due to historical deforestation. This historical scenario compared distributed river flow (Q) generated from pre-industrial land cover with river flow derived from contemporary landcover.

Answer 4

``` • Urbanisation increases the amount of impermeable surfaces, which reduces the amount of infiltration and percolation. • The high drainage density associated with the network of drains, gutters, sewers and flood relief channels, quickly carries water into river channels, thereby increasing channel storage. Example Land Cover/Use Change: Urbanisation • Moreover, water quality is affected by urbanisation. Overland flow in urban areas picks up suspended solid particulate matter and dissolved chemicals, from motor exhausts for example, making water quality poor, and in some cases it may even be toxic. ```

Answer 5

• Irrigation has increased dramatically to sate the increasing demand for food for a burgeoning population, and particularly in drier regions, where rainfed agriculture is not feasible • Drier regions also require more irrigation water because of the higher PET rates • This requires withdrawing water from rivers and groundwater, causing rivers to dry and resources to be depleted. • At the same time, it can cause enhanced surface runoff and leaching of nutrients, which leads to increased use of fertiliser, with potential for pollution of waterways • Another problem is that salt is brought up from lower levels (salination).

Answer 6

* Direct impacts on local water –increased ET * Potential changes in runoff and recharge * Indirect impacts on regional water via recycling bb

Answer 7

• The use of water-bearing rocks – aquifers – to supply water is widespread. • Problems: if the use of groundwater exceeds the recharge of groundwater, the water table drops. • One of the main problems of groundwater abstraction is in coastal areas, namely saltwater intrusion. Salt-water intrusion is severe along the Mediterranean, Black Sea and Baltic coasts. • Wetlands are also becoming scarce, and many rivers are drying up, with impacts on aquatic ecosystems, as well as economic functions such as water supply and navigation. • Groundwater pollution is difficult to treat. Groundwater is filtered slowly – sometimes over centuries and millennia – and its temperature and oxygen levels are lower than surface waters.

Answer 8

Simplified version of a global groundwater resources map highlighting the locations of regional aquifers systems bb

Answer 9

• Land subsidence is a gradual settling or sudden sinking of the Earth's surface owing to subsurface movement of earth materials. • The principal causes are aquifer-system compaction, drainage and decomposition of organic soils, underground mining, oil and gas extraction, hydrocompaction, natural compaction, sinkholes, and thawing permafrost (National Research Council, 1991). • Effects include damage to buildings and roads/canals, increased flood risk in lowlying areas, and lasting damage to groundwater aquifers and aquatic ecosystems.

Answer 10

Among the world's largest groundwater basins, the Indus Basin aquifer of India and Pakistan, is a source of fresh water for millions of people northern India, which includes the nation’s breadbasket of wheat and rice production Scientists have estimated that northern India is depleting groundwater at a rate of 54 billion cubic meters per year, a volume that could support a subsistence-level diet for some 180 million people. In addition to the breadbasket states of Punjab and Haryana in the northwest, groundwater levels are falling extensively across India in Andhra Pradesh, Gujarat, Maharashtra, Rajasthan and Tamil Nadu. More than 15 percent of India’s food is being produced by mining groundwater • The electric pump - a symbol for India’s “resourcedraining, energy-wasting, treasury-tilting” farm policies . • The agriculture sector makes up 19 percent of India’s electricity consumption, most of it to power India’s 20 million electric pumps that pull water from deep in the ground. • Provision of free or cheap electricity and water to India’s poor population has led to the proliferation of millions of electric water pumps that deplete groundwater supplies in the fertile northern graingrowing region. • So much of the country’s electricity is used to power water pumps that India’s coal-fired electrical sector and government-owned coal mines cannot keep up with demand

Answer 11

• With 54 percent of India’s total area facing high to extremely high stress, almost 600 million people are at higher risk of surface-water supply disruptions. • Note, in particular, the extremely high stress area blanketing Northwest India. The region is India’s breadbasket, producing the majority of the country’s water intensive rice and wheat stocks • Groundwater levels are declining across India. Of the 4,000 wells showing statistically significant trends, 54 percent dropped over the past seven years, with 16 percent declining by more than 1 meter per year. • Farmers in arid areas, or areas with irregular rainfall, depend heavily on groundwater for irrigation. The Indian government subsidizes the farmers’ electric pumps and places no limits on the volumes of groundwater they extract, creating a widespread pattern of excessive water use and strained electrical grids. • Northwestern India again stands out as highly vulnerable. Of the 550 wells studied in the region, 58 percent have declining groundwater levels.

Answer 12

``` The discharge of many large rivers has indisputably been affected by water management, especially dam construction - but also within-basin diversions for other beneficial uses such as irrigation and municipal and industrial water supply, trans-basin diversions, land use, … ```

Answer 13

Since construction of the High Aswan Dam almost all the Nile River’s discharge is now either diverted for irrigation or lost to reservoir evaporation. Flows of the Syr Darya River have declined greatly because of irrigation diversions upstream (one of the major tributaries of the Aral Sea). In contrast, the flow of the Burntwood River increased by a factor of four following an upstream diversion into the river basin in the 1970s for hydro- power production. Observed flow of the Columbia River after construction of large reservoirs, as well as ‘naturalized’ discharge and projected effects of climate change on the naturalized flows by 2050 bb

Answer 14

• Two of the largest consequences of dam construction (and other management) are river fragmentation and river regulation. * These can be represented by the * River Fragmentation Index (RFI) and * River Regulation Index (RRI) * Quantifying these types of indices is useful in characterizing effects on * water resources * biodiversity, * ecosystem functioning, and * fluvial geomorphology

Answer 15

• A measure of river fragmentation by barriers on structural connectivity per basin and is defined as the volume of flow separated by the barrier. • Where Qannual,ds is the annual discharge downstream of the dam and Qannual is the annual discharge for the whole basin • The RFI of an unfragmented river network is 0%, with each subsequent dam increasing the value to a maximum of 100%. • A single dam in a previously undisturbed network leads to greatest fragmentation if it splits the network into two equal volume fragments, in which case the RFI increases to 50%.

Answer 16

• The DOR is the proportion of a river’s annual flow volume that can be withheld by a reservoir or a cluster of reservoirs upstream of the reach and is calculated for each reach of the network. ``` • Where C is the capacity of the reservoir (km3) and Qannual is the annual flow of the river (km3) ``` • A high DOR indicates an increased probability that substantial discharge volumes can be stored upstream in a given year for future release. • The RRI is an extension of the degree of regulation (DOR) and provides a quantitative proxy of how strongly a river may be affected by alterations to its natural flow regime due to upstream dam operations. • RRI (%) is calculated by first weighting the DOR value of each individual reach with its corresponding river volume, and then averaging the results for the entire basin to quantify full-basin impacts in a single index.

Answer 17

The degree of regulation is the ratio between total upstream storage capacity and the annual average discharge of a river reach. DOR can be used as an indicator of impact on downstream flows

Answer 18

We can combine the RFI and RRI to see the global picture of impacts of dams • Green indicates basins with low fragmentation and low regulation • Blue indicates basins with low fragmentation but severe regulation • Yellow indicates basins with severe fragmentation but low regulation • Red indicates basins with severe fragmentation and severe regulation *48% of river volume is moderately to severely impacted by either flow regulation, fragmentation, or both. Assuming completion of all dams planned and under construction in our future scenario, this number would nearly double to 93%, largely due to major dam construction in the Amazon Basin.

Answer 19

1. Carry out experiments using hydrological models with and without water management 2. Carry out experiments using climate models (regional) using different land covers 3. Impacts can also be detected by looking at river flows pre and post change

Answer 20

``` CD5 covers the Central Valley which is heavily irrigated. So, human management has a large influence on river flows and drought. ``` bb

Answer 21

Comparison of standardized deficit volumes with (left) pristine condition and with (middle) transient human water consumption, and (right) relative contribution of human water consumption (%) for major drought events over (a)–(c) North America (2002), (d)–(f) Europe (2003), and (g)–(i) Asia (2001). bb