Hydrology

What is hydrology?

Hydrology is the movement, distribution (quantity), and quality of water on Earth and its cycle through the environment. Hydrology is the process that transports contaminants, either dissolved in the water or as suspended particulate material. As all contaminants are transported by water, the pathway water takes across or through the land surface influences the type and magnitude of contaminant transport to waterways.

The water cycle.

What is surface water?

Surface water includes all the water above ground, such as rivers, streams, lakes, ponds and wetlands.

What is a surface water catchment?

A catchment area is a term used to describe an area which collects each drop of water (rain) that falls into that area and which eventually ends up in the same river or stream going to the sea.  

A catchment (also known as a drainage basin) is a basin shaped area of land bounded by natural features, such as hills or mountains, from which surface and subsurface water flows into streams, rivers, and wetlands. Water flows into, and collects in, the lowest areas in the landscape.

Example of a surface water catchment for the Waikato River. The Waikato River catchment (purple) includes the surface water catchment for Lake Taupo (orange). The Pokaiwhenua stream (red) is a tributary to the Waikato River and joins the river at Lake Karapiro. There are many tributaries which join to the Waikato River on its way to the sea at Port Waikato.

Why is it important to know your catchment?

No catchment is exactly like another. Each has a different size, shape, drainage pattern and features that are determined by natural processes, particularly geology and climate.

The geology of your catchment will influence many of its characteristics, from the stability of the streambanks and streambed to the natural chemistry of the water. Climatic processes and flowing water erode and shape the land. As rocks are broken down into smaller pieces they can be transported in the flow. Fine materials are transported as sediment throughout the catchment. Weathered rock and organic matter make up the soils that blanket the landscape. Soils have different textures, mineral content, structure and drainage properties. The nature of the soils in your catchment will have a key role in deciding how much water runs off the land and how likely the land is to erode.

The concept of a catchment is useful, because it is the scale on which many parts of the landscape work. The soil, plants, animals and water all function together in a catchment – anything that affects one of these will also have an impact on the others. Therefore, if you want to improve water quality in a river, you need to know the contributing area to that river and the factors that are affecting water quality.

What is a drainage network?

The system of streams which transport water, sediment, and other material from a catchment is called a drainage network.

Tributaries are small feeder streams that empty into larger streams or rivers. The catchments of tributaries are referred to as sub-catchments. Large catchments are often made up of a number of smaller sub-catchments.

Example of a drainage network. The stream order increases when two waterways of the same order converge.

What is stream order?

The branching nature of a river and its tributaries are known as the stream order (or Strahler order). It is used to define the size of a stream based on the hierarchy of the tributaries flowing to a point of interest. If two tributaries of the same order combine, the next ‘order’ in the sequence is used to define the stream order (i.e. 1+1 = 2, 4+4 = 5). A lower order stream joining a higher order does not change the stream order (i.e. 1+3 = 3).

Whatever happens in each of the smaller streams affects the overall wellbeing of the main waterway. Therefore, to make improvements in water quality in a larger stream our focus should be on improving the low order streams.

What pathways does water take across or through the landscape?

Water is the mechanism that transports all contaminants from the land. Therefore, the pathways that water takes to drain from the land are critical to understanding what contaminants are likely to be mobilised and how to minimise losses. These pathways can be predicted from the topography, soil and geological properties. The climate and seasonality of precipitation also plays a critical role in when contaminants are lost during the year.

Example of hydrological gradients across the landscape.

The main hydrological flow pathways are:

  • Deep drainage through the soil zone into the underlying aquifer (groundwater)
  • Lateral drainage occurs through the soil along the contact with shallow bedrock or slowly permeable layers in the soil zone
  • Artificial drainage is used to improve drainage where soils have either poor drainage or slow permeability in agricultural areas
  • Overland flow via surface runoff
  • Natural bypass flow when high clay soils are dry and cracked

Flow pathways water takes to enter a waterway.

What is deep drainage?

Deep drainage or groundwater recharge is where water moves downward through the soil profile. Recharge is the primary method through which water enters an aquifer. Deep drainage and lateral drainage through the soil are key transport pathways for phosphorus and microbes.

What is lateral flow?

Lateral flow is common in areas where there is shallow bedrock, such as in hill country. We often see seeps and springs occurring where lateral drainage flow paths converge with the land surface. Lateral flow also occurs close to waterways.

What is artificial drainage?

Artificial drainage speeds up the lateral flow through the soil. It reduces the moisture in soil and thereby increases the amount of air which provides conditions for optimal growth of crops. Artificial drainage includes surface ditches, in addition to subsurface, mole and tile drains. Open ditch drainage is typically used to lower the water table. Open ditches in conjunction with subsurface drainage are used to improve drainage through poorly drained soil.

What is overland flow?

Overland flow (also known as surface or surficial runoff) is the flow of water that occurs when excess rainwater, meltwater, or water from other sources flows over the  surface of the ground. This might occur because soil is saturated to full capacity, because rain falls more quickly than soil can absorb it, or because the land surface is impervious (e.g. bare rock and roads). Surface runoff is a major component for the transport of contaminants from the landscape. It is the primary agent in soil erosion by water.

Overland flow transports water, solutes and particulates from the land surface to a surface water body.

Example of overland flow occurring in a paddock. Image credit: DairyNZ
Example of urban surface runoff (overland flow). Image credit: Envivo Ltd.

What is Natural Bypass?

Soils with a high clay content can crack when they are dry. These cracks become a preferential pathway for water to drain through, however as the drainage is rapid, there is minimal contaminant removal from the soil zone. High concentrations of bacteria, such as E. coli, in groundwater is typically from this transport mechanism. In areas where artificial drainage is also present, these cracks can connect to the subsurface drainage network. Contaminants can directly enter a surface water body in these situations.

What is leaching?

Leaching is the transport of dissolved species through the soil into an aquifer (groundwater). The loss of nitrogen, in the form of nitrate, is an example of a common environmental contaminant transported by leaching. Leached contaminants are typically transported by deep and lateral (including artificial) drainage.

What is groundwater?

Groundwater is the water present beneath the Earth's surface in pore spaces and in the fractures of rock. Rock or unconsolidated material, such as gravel, is called an aquifer when it can yield a usable quantity of water. We access this water through drilling wells or bores.

Water under the Earth’s surface is called groundwater.

What is an aquifer?

An aquifer is an underground layer of water-bearing rock. Water in an aquifer is called groundwater. Aquifers are permeable, meaning they have openings that liquids and gases can pass through. Sedimentary rock such as sandstone, as well as sand and gravel, are examples of water-bearing rocks. The top of the water level in an aquifer is called the water table. A water table describes the boundary between water-saturated ground and unsaturated ground. Below the water table, rocks and soil are full of water.

An aquifer fills with water from rain or melted snow that drains into the ground. In some areas the water passes through the soil on top of the aquifer and in other areas it enters through joints and cracks in rocks (fractured rock aquifer). The water moves downward until it meets less permeable rock. 

Aquifers act as reservoirs for groundwater. Water from aquifers drain to streams and sustain the baseflow of a stream or river. Sometimes groundwater is discharged at the land surface as a spring or wetland (seep). 

Groundwater accounts for roughly 30% of New Zealand’s water usage and is essential for environmental and social well-being and economic productivity. Wells (or bores) drilled into aquifers provide water for drinking, agriculture, and industrial uses. Groundwater in an aquifer can dry up when wells are drained faster than nature can refill (recharge) them. Because aquifers fill with water that drains from the land surface, they can be contaminated by any chemical or toxic substance found on the surface.

What is an unconfined aquifer?

There are two types of aquifers. An unconfined aquifer is covered by soil and permeable rock and can receive water from the land surface directly above. The water table of an unconfined aquifer rises or falls depending on the amount of water entering and leaving the aquifer. Relatively, this water is young and hydrologically connected to the surface water network. This means the activities we do on the land can have a big impact on the quality of this water source. It is especially important as this is the source of water to streams and rivers when they are at their lowest flow (base flow) over the summer months.

What is a confined aquifer?

A confined aquifer lies between two layers of less permeable rocks and is filled with water over a much longer time period. Water trickles down through cracks in the upper layer of less permeable rock, and an aquifer that is partially disconnected from the land surface directly above occurs.

What is groundwater recharge?

Groundwater recharge occurs where water moves downward through the soil profile. Recharge is the primary method through which water enters an aquifer and is also known as deep drainage.

Is groundwater included in Physiographic Environments?

All water that is hydrologically connected to the stream contributes to the water quality, therefore is included in our physiographic environment classification. The water in the river or stream after a period of dry conditions is predominantly sourced from groundwater. As the catchment becomes wetter, contributions from the soil zone (lateral and artificial drainage) increase. During a storm event, overland flow can directly enter the waterway.

Groundwater connections to surface water.