Text/Hanlin Reading History
Editor/Hanlin Reading History
introduction
Alpine meadow is a typical landscape of the Qinghai-Tibet Plateau at an altitude of 3000 ~ 5500 meters, in addition to providing fodder for grazing animals, alpine meadow formation meadow can stabilize the surface and protect the soil below from water erosion.
In recent years, large areas of the plateau have shown signs of continuous degradation of turf and erosion of the underlying soil.
In this study, we simulated the meadow-causing rupture mechanism and developed a process-based land degradation model of the Tibetan Plateau slope.
The model considers four possible causes: turf cracking, water flow concentration in the crack, crack expansion caused by scouring, and plate flow erosion.
As expected experimentally, soil erosion increases with slope and watershed area (so stronger erosion is observed at relatively steep downhill sites).
The model simulation results show that the landscape of the Qinghai-Tibet Plateau is susceptible to turf degradation and soil erosion under a representative set of reasonable soil and hydrological conditions.
Once polygonal fissures form, water currents widen the fissures until the landscape is completely barren, at which point the flake eventually erodes the mineral soil, leaving a highly degraded landscape.
Our model shows that the stability of K. pygmaea turf depends on slope and precipitation, and the time for erosion of the bottom turf varies from decades to centuries.
Therefore, we can think that these straw mats may be bistable, with a clear degenerative mechanism, but no clear regeneration mechanism.
I. Speculation and current status of causes of erosion degradation of alpine meadows
The alpine meadows (Kobresia pygmaea) are the world's largest alpine ecosystem, almost ubiquitous on the Tibetan Plateau, covering a total area of 450,000 km2.
They are mainly found in alpine pastures ranging from 3,000 to 5,500 meters above sea level, in areas with an average annual rainfall of 200-1,000 mm/year.
Alpine meadows are undergoing changes due to warming and potential overgrazing, which play a role at different stages of the degradation process.
To date, more than three-quarters of green meadows have been discontinuously covered, with soil cracks at different stages of degradation and erosion ditches of different widths dividing the patchy landscape.
Traditional speculation on the causes of degradation suggests that livestock (yaks) play a major role in patch erosion.
In fact, there are scientific studies that prove that degradation is not related to yak trampling, because the degradation of meadows continues, despite the current decrease in grazing density.
As a result, scientists have hypothesized that global warming (rather than grazing) has reduced the quality of pasture in these grasslands.
Alpine grass forms a strong and continuous turf mat, about 10 cm thick, the turf remains frozen in winter, thaws in late spring and summer, and often freezes at night during part of spring and summer, depending on the altitude.
The turf mat isolates the underlying soil, thereby reducing the frequency of nocturnal soil freezing during the growing season. , turf mats protect and stabilize the underlying (often thin) mineral soil layer.
Turf has been formed for at least the last few thousand years, probably a reaction to the formation of early grazing of livestock on the plateau, which also shows from the side that if the turf does not crack, it is stable in terms of erosion.
The turf is cut into pseudohexagons (usually ≈ 0.5-2 m in diameter) by stretch cracking, which is caused by drying processes associated with climate change.
The cutting of cracks by runoff water and other processes leads to the gradual erosion of the edges of these polygons, increasing the spatial extent of eroded (bare ore) areas.
The degradation process of alpine grass turf is shown in the figure below.
As these erosion lines widen, the cracks connect, increasing the connectivity of water and sediment transport pathways, which increases flow and begins the erosion process of the mineral soil underneath, accelerating the destruction of turf.
The hillside became lawn bases and blocky mosaics that eventually disappeared due to erosion, leaving behind bare mineral soils that were vulnerable to erosion.
At the bottom of the valley one can observe the cracking of the turf into hexagons; Similarly, soil erosion due to erosion of hexagons can only be observed on slopes.
The purpose of this study was to simulate K. Fragmentation and erosion of pygmaea turf to help limit the degradation timescale of these soils under different conditions and to estimate changes in erosion rates during their degradation.
Second, simulation of erosion degradation state of alpine meadow grass under different rainfall conditions
The model is used to represent the state of turf degradation observed in the field, interactive data language (IDL) code is provided in the supporting information, and the model is initially conditioned for hexagonal cracking of slope turf.
The stream of sheets flowing through the turf concentrates into the cracks, initially assumed to be 1 cm wide, and these cracks are all connected, and erosion occurs when the shear stress exerted by the water flow exceeds the shear strength of the underlying sediment.
The rate of erosion within the channel depends on the flow rate of water Q(w) through each channel, which is calculated as
Since rainfall depth P is related to rainfall duration D (nonlinear), especially in the case of extreme events, its intensity usually decreases with duration - P and D are modeled as correlated random variables):
For each rainfall event, D is randomly sampled with the following average index distribution:
where P(*) is the conditional mean of P for a given D and m is an exponent of the depth-duration scale law, which ranges from 0.3-0.5 in most climates, here equal to 0.4:
where b is the channel width and h is the flow depth: b and h are both dynamic variables; The change in b is related to the erosion process, while the change in h is due to different flow values due to rainfall events of different magnitude, and the value of h is calculated by solving the Manning equation.
Based on simple geometric considerations, it can be shown that the drainage area at each connecting node A is equal to nA(1), where A(1) is the area of a single hexagonal patch (i.e. the hexagon of the lawn plus half of the surrounding channel or the area of widening the prehexagon), and l is the length of the prehexagonal side of the channel widening (here 1 meter).
Using Manning's uniform flow equation (Q(w) = n(−1)bhR(h)(2/3)S(1/2)), the flow depth h in the channel is calculated as a function of Q(w) and slope S, and the value of Manning's roughness parameter n is 0.03 S m (−1/3), where R(h) is the hydraulic radius:
The shear stress τ(s) applied by the water flow is calculated as
where ρ(w) is the density of water and g is the acceleration due to gravity.
Erosion occurs when τ(s) exceeds the critical value τ(c), depending on the soil cohesion c(s), the additional cohesion of the turf σ(s):
where ρ(s) is the density of mineral soil or gravel particles (2.5 g/cm(3)) and d is the average particle size of erodable soil under turf.
Soil cohesion c(s) varies with texture change, in the case of non-structural clay can be in the range of 0-110 Pa, the cohesion effect of turf is related to root strength and density, between σ(g) = 2.8 ~ 11.2 kPa.
In this case, σ (g) is taken as zero, since erosion occurs in the organic layer of clay below the base layer, and in the event of duration D, the thickness of the sediment removed when the sides of the river channel widen to the height of the water depth δ can be determined by the following formula:
where ε is the intrinsic erosion of the soil, usually in the range of 1 × 10(−5)-5 × 10(−5)kg·m(−2)·s(−1). After each erosion event, the channel is widened by 2 δ (i.e. 1 δ on both sides). In erosion events, the rate of channel widening and hexagonal narrowing can be expressed as:
It is assumed that the destruction that occurs on the turf will also lead to the loss of the overlying turf through some undefined mass waste process, frost heaving and trampling of the edges by yaks can cause the turf above to collapse.
In general, the slope length factor and slope steepness factor in RUSLE, L(RUSLE) and S (RUSLE) are calculated using the same slope and the distance w≥25 cm from the bottom of the slope to the bottom hexagon.
The soil erosion factor K (RUSLE) was calculated by the geometric mean particle size (D(g) = 0.26 mm) corresponding to the above arithmetic mean particle size (D = 0.86 mm).
For each rainfall event, the maximum 30-minute intensity is calculated as the average intensity (P/D) multiplied by the multiplier c:
where q is the non-uniformity parameter, which is used to calculate RUSLE storm erosion force EI and rainfall runoff intensity factor R (RUSLE).
Unified settings for the coverage management factor C (RUSLE) with the support practice factor P (RUSLE).
Each iteration of the model represents a rainfall event with a daily rainfall probability of 7.4% and a summer rainfall frequency of ~0.30 rainfalls/day.
Precipitation outside of summer is considered solid precipitation and seeps into the soil without erosion as it melts.
Over time, the main output of the model is the hexagon width w, from which the hexagon can be visualized.
As an indicator of the rate at which the lawn crushing process occurs, we calculate the "degradation time" T(d), i.e. the time when the maximum width of the lowest hexagon on the slope is less than 25 cm.
3. Observation and analysis of hexagonal crack data in alpine meadow erosion model
Although the representation of erosion in the model is overly simplistic, it reproduces several features expected by the system.
The degradation of hexagonal lawns is most severe at the foot of the slopes, and after a few years, some of the slopes have no turf at all.
Based on our model, the hexagonal initial erosion occurs rapidly after several particularly strong rainfall events.
After two to three years, the hexagon at the bottom of the 150-meter slope may be 60% smaller than its original size, and in about 100-200 years, the turf at the bottom of the slope may disappear.
The time it takes for the hexagon to disappear depends on the length of the slope, with longer slopes experiencing shorter degradation times due to higher water velocity.
For low slopes (<0.08), slopes with a maximum slope of 200 (about 300 m) may be completely degraded in more than 800 years, while steeper slopes may be completely degraded at the bottom within a few decades.
Slope erosion begins and increases non-linearly after the erosion of the bottom turf, and it takes 60-70 years for slope erosion at the bottom of the 300 m slope to begin.
summary
Our findings show that once a lawn is significantly degraded, the amount of erosion will reach 0.1 mm per year over a period of two centuries.
This comparison does not show that turf degradation controls the entire erosion process on the plateau, on the contrary, in areas where degradation usually occurs, the loss of turf may lead to similar erosion rates observed elsewhere.
Our model suggests that once hexagonal erosion is formed, turf degrades relatively rapidly in the face of erosion, and once cracking triggers this process, the entire landscape will undergo an inevitable degradation trajectory until the entire turf disappears completely.
It is not clear whether alpine grass turf can be re-established on poor and degraded soils, and if reconstruction takes more than decades, the process will be irreversible on a generational timescale.
The ecosystem may exist in two possible stable structures – intact turf cover and bare mineral soil, but the turf fragments observed in many places today represent only a transient state.
Kobresia pygmaea, similar to an ecosystem engineer, creates and maintains its own habitat by promoting soil stability, creating a positive feedback relationship between green grassland cover and subsoil.
Positive feedback mechanisms in ecosystems are known for their ability to induce bistable dynamics, meaning that the system is stable in two different states, namely stable grassland soil cover and degraded barren soil.
If this is indeed the case, the grass cover of Alpine Songgrass will have limited resilience and will easily transition to a stable state of degradation, which may be irreversible or take a long time to recover.
This shift will be associated with the loss of important ecosystem services, including grazing by major livestock yaks, the major ecosystem at the Earth's "third pole," and these meadows are critical to regulating land-atmosphere interactions in this critical climate region.
The irreversible loss of grasslands and their replacement by bare slopes represent significant changes in surface energy, water and material fluxes.
As the Tibetan Plateau is expected to experience dramatic climate change (precipitation will increase by about 12% and temperatures by about 4.1°C by the end of the century), the dynamics of these systems are essential to set a baseline for future research and inform modelling from the hillsides to the global scale.
bibliography
[1] Zhao Changliang, "Analysis of Restoration Technology of Degraded Grassland in Subalpine Meadows in Yunding Mountain, Shanxi"
[2] Yang Jun, "Plant communities and soil nutrients at different stages of degradation in alpine grass alpine meadows on the Qinghai-Tibet Plateau"
[3] Liu Yongtao, "Analysis of aboveground biomass and suitable livestock carrying capacity during the restoration of degraded subalpine meadows"
[4] Sheng Zhilu, "Study on the functional diversity of plant communities in alpine meadows on degraded gradients"
[5] Zhongxin Feng, "Effects of grassland cultivation measures on vegetation characteristics of severely degraded subalpine meadows"