# What is density of soil

14.03.2021 By Gokasa

Drainage density

9 rows · Soil density is the relation between the mass and the volume of a dry soil sample. In its. An average value for particle density is g/cm 3. This means that a soil particle that is 1 cubic centimeter in volume weighs g. In comparison, water has a density of 1 g/cm 3, and.

Soil weight is most often expressed on a soil volume basis rather than on a particle basis. Bulk density is defined as the dry weight of skil per unit volume of soil.

Bulk density considers both the solids and the pore space; whereas, particle density considers only the mineral solids. Figure 2. For our ideal soil, one-half of it is solids, and one-half is pore space. Using our example of a 1 cm 3 volume, the ideal soil would have 0. Soli space filled with air weighs 0 g. Organic matter is a very small portion of the solids, so it is usually ignored in this calculation.

The mineral solids would weigh 1. For practice, densiy a box of undisturbed soil from the field. The box has dimensions of 2. The volume of the box can be determined by multiplying the height of the box times its width and how to clear email cache depth. The wet soil in the box weighed g. The dry soil weighed g. Now calculate the bulk density. Your answer should be 1. In this calculation, you did not have to use the particle density because the weight of soil in the box was already known.

Bulk density of the surface soil is lowest in the spring immediately after soils have thawed and before field operations have densihy. Each field operation compacts soil beneath the tires. If soils are wetter than field capacitybulk density may increase. However, if soils are dry, bulk density is not affected much.

Root growth, in general, starts to be restricted when the bulk density reaches 1. Tillage can increase bulk density if it breaks down aggregates and allows soil separates to pack more tightly. Adding organic material decreases bulk density because organic material has a lower bulk density.

However, additions are typically so small in proportion to the weight of soil that they do not markedly influence bulk density except at the soil-atmosphere interface.

Bulk density is also important because it tells us about the porosity of a soil. Bulk Density Soil weight is most often expressed on a soil volume basis rather than on a particle basis. Previous Page Next Page.

Unit Weights of Soil

It depends upon the accumulative densities of the individual inorganic and organic constituents of the soil. Generally in the normal soils the particle density is grams per cubic centimetre or mega grams per cubic metre. A guide to Soil Types has been provided by StructX and additional information has been provided below. Notation. ? = bulk density (the ratio of the total mass to the total volume), ib/ft 3 or kg/m 3; ? sub = submerged bulk density (the ratio of the total submerged mass to the total volume), ib/ft 3 or kg/m 3. Density = Mass / Volume. Soil density is expressed in two well accepted concepts as particle density and bulk density. In the metric system, particle density can be expressed in terms of mega grams per cubic meter (Mg/m3). Thus if 1 m3 of soil solids weighs Mg, the particle density is Mg / m3 (since 1 Mg =1 million grams and 1 m3 =1 million cubic centimeters) thus particle density can also be .

Drainage density is a quantity used to describe physical parameters of a drainage basin. First described by Robert E. Horton, drainage density is defined as the total length of channel in a drainage basin divided by the total area, represented by the following equation:. Drainage density depends upon both climate and physical characteristics of the drainage basin.

Soil permeability infiltration difficulty and underlying rock type affect the runoff in a watershed; impermeable ground or exposed bedrock will lead to an increase in surface water runoff and therefore to more frequent streams.

Rugged regions or those with high relief will also have a higher drainage density than other drainage basins if the other characteristics of the basin are the same. When determining the total length of streams in a basin, both perennial and ephemeral streams should be considered. Ignoring ephemeral streams in the calculations does not consider the behavior of the basin during flood events and is therefore not completely representative of the drainage characteristics of the basin.

Drainage density is indicative of infiltration and permeability of a drainage basin, as well as relating to the shape of the hydrograph. High drainage densities also mean a high bifurcation ratio. Drainage density can be used to approximate the average length of overland flow in a catchment. Horton used the following equation to describe the average length of overland flow as a function of drainage density: [2].

Considering the geometry of channels on the hillslope, Horton also proposed the following equation:. A drainage basin can be defined by three elementary quantities: channels, the hillslope area associated with those channels, and the source areas. Channels of lower orders combine to form higher order channels. The associated hillslope areas are the hillslopes that slope directly into the channels.

The source areas are concave regions of hillslope that are associated with a single channel. Source areas and the hillslope areas associated with channels are differentiated by source areas draining through the channel head, while the associated hillslope areas drain into the rest of the stream. Bras et al. Channel formation is a concept intimately tied to the formation and evolution of a drainage system and influence the drainage density of catchment.

The relation they propose determines the behavior of a given hillslope in response to a small perturbation. They propose the following equation as a relation between source area, source slope, and the sediment flux through this source area:.

Where F is the sediment flux, S is the slope of the source area, and a is the source area. The right-hand side of this relation determines channel stability or instability. If the right-hand side of the equation is greater than zero, the hillslope is stable, and small perturbations such as small erosive events do no develop into channels.

Conversely, if the right-hand side of the equation is less than zero, Bras et al. In this sense, "unstable" is not used in the sense of the gradient of the hillslope being greater than the angle of repose and therefore susceptible to mass wasting, but rather fluvial erosive processes such as sheet flow or channel flow tend to incise and erode to form a singular channel. Drainage density relates to the storage and runoff terms. Drainage density relates to the efficiency by which water is carried over the landscape.

Water is carried through channels much faster than over hillslopes, as saturated overland flow is slower due to being thinned out and obstructed by vegetation or pores in the ground. Consequently, a drainage basin with a relatively higher drainage density will be more efficiently drained than a higher density one.

Conversely, precipitation entering a lower drainage density basin will take longer to exit the basin due to travelling over the slower hillslope longer. In his paper on drainage density and streamflow, Charles Carlston found that baseflow into streams is inversely related to the drainage density of the drainage basin:. This equation represents the effect of drainage density on infiltration.

As drainage density increases, baseflow discharge into a stream decreases for a given basin because there is less infiltration to contribute to baseflow. Gregory and Walling found that the average discharge through a drainage basin is proportional to the square of drainage density:.

This relation illustrates that a higher drainage density environment transports water more efficiently through the basin. The increased infiltration results in a decreased surface runoff runoff according to the water balance equation. These two equations agree with each other and follow the water balance equation. According to the equations, a basin with high drainage density, the contribution of surface runoff to stream discharge will be high, while that from baseflow will be low.

Conversely, a stream in a low drainage density system will have a larger contribution from baseflow and a smaller contribution from overland flow. The discharge through the central stream draining a catchment reflects the drainage density, which makes it a useful diagnostic for predicting the flooding behavior of a catchment following a storm event due to being intimately tied to the hydrograph. Water flows significantly slower over hillslopes compared to channels that form to efficiently carry water and other flowing material.

On the hydrograph, the peak is higher and occurs over a shorter range. The timing of the hydrograph in relation to the peak of the hyetograph is influenced by the drainage density. Conversely, the water entering a low drainage density basin will, on average, have to travel a longer distance over the low velocity hillslope to reach the channels. As a result, the water will require more time to reach the exit of the catchment. The lag time between the peak of the hyetograph and the hydrograph is then inversely related to drainage density; as drainage density increases, water is more efficiently drained from the basin and the lag time decreases.

Another impact on the hydrograph that drainage density has is a steeper falling limb following the storm event due to its impact on both overland flow and baseflow. In higher drainage systems, the overland flow reaches the channels quicker resulting in a narrower spread in the falling limb. Baseflow is the other contributor to the hydrograph. The peak of baseflow to the channels will occur after the quick-flow peak because groundwater flow is much slower than quick-flow.

Because the baseflow peak occurs after the quick-flow peak, the baseflow peak influences the shape of the falling limb. During a storm event in a high drainage density basin, there is little water that infiltrates into the ground as infiltration because water spends less time flowing over the surface in the catchment before exiting through the central channel.

Because there is little water that enters the water as infiltration, baseflow will contribute only a small part to falling limb. The falling limb is thus quite steep. Conversely, a low drainage system will have a shallower falling limb.

Montgomery and Dietrich [3] determined the following equation for drainage density by observing drainage basins in the Tennessee Valley, California:.

R 0 , the average precipitation term, shows the dependence of drainage density on climate. With all other factors being constant, an increase in precipitation in the drainage basin results in an increase in drainage density. The equation also shows the dependence on the physical characteristics and lithology of the drainage basin. Materials with a low hydraulic conductivities, such as clay or solid rock, [6] would result in a higher-drainage density system. Because of the low hydraulic conductivity, there is little water lost to infiltration and that water exits the system as runoff and can contribute to erosion.

In a basin with a higher vertical hydraulic conductivity, water more effectively infiltrates into the ground and does not contribute to saturated overland flow erosion, resulting in a less developed channel system and therefore lower drainage density. Charles Carlston [8] determined an equation to express the mean annual flood runoff, Q2. Carlston found a correlation between the two quantities when plotting data from 15 drainage basins and determined the following equation:. Where Q is in units of cubic feet per second per square mile and D d is in units of inverse miles.

From that equation, it is concluded that a drainage basin will adjust itself through erosion such that this equation is satisfied. The presence of vegetation in a drainage basin has multiple effects on the drainage density.

Vegetation prevents landslides [11] in the source area of a basin that would result in channel formation as well as decrease the range of drainage density values regardless of soil composition.

Vegetation stabilizes the unstable source area in basin and prevents channel initiation. Because the soil is held in place, it is less prone to erosion from those physical methods. The erosional processes that may lead to channel initiation are prevented.

The increased soil strength also protects against surface runoff erosion, which hinders channel evolution once it has begun. At the basin scale, there are fewer channels in the basin and the drainage density is lower than an unvegetated system. The effect of the vegetation on decreasing the drainage density is not unbounded though. At high vegetative coverage, the effect of increasing the coverage diminishes.

This effect imposes an upper limit to the total reduction in drainage density that vegetation can result in. Vegetation also narrows the range of drainage density values for basins of various soil composition.

Drainage density is related to the ease at which channels can form. Coarse-grained sediment like sand would have a higher hydraulic conductivity and are predicted by the equation to form a relatively higher drainage density system than a system formed by finer silt with a lower hydraulic conductivity. Forest fires, both natural and unnatural, destroy some or all of the existing vegetation, which removes the stability that the plants and their roots provide. Newly destabilized hillslope in the basin is then susceptible to channel formation processes, and drainage density of the basin may increase until the vegetation grows back to the previous state.

The type of plants and the associated depth and density of the plant roots determine how strongly the soil is held in place as well as the intensity of the forest fire in killing and removing the vegetation. Computer simulation experiments have validated that drainage density will be higher in regions that have more frequent forest fires.

Drainage density may also be influenced by climate change. Langbein and Schumm 9 propose an equation for the rate of sediment discharge through catchment as a function of precipitation rate:. The graph of this equation has a maximum between 10 and 14 inches and sharp declines on either side of the peak.

At lower effective rainfalls, sediment discharge is lower because there is less rainfall to erode the hillslope. At effective rainfalls of greater than inches, the decrease in sediment yield is interpreted to be the result of increasing vegetation cover. The badlands of Caineville, Utah are often cited as a region of extremely high drainage density. The region features steep slopes, high relief, an arid climate, and a complete absence of vegetation. The regions below the angle of repose, however, are still generally at a significant angle, and hillslope diffusion, according to the following relation, is still a significant source of erosion:.

The range of drainage densities in the Caineville Badlands illustrates the complicated nature of drainage densities in low-precipitation environments. This occurs until a threshold is reached at a higher relief ratio, when increase the slope ratio is accompanied by a decrease in drainage density. At a higher slope, the erosion is faster and more efficiently funneled through fewer channels. This qualitative topographic map of a section of a section of the Caineville Badlands shows the extensive drainage network in the arid environment.

The image of the Caineville Badlands displays the lack of vegetation and numerous channels. The Caineville Badlands are located in an arid environment, receiving an average of mm of precipitation per year.