CN110188404B - Method for determining forest and grass vegetation coverage rate threshold capable of restraining sand production in river basin - Google Patents

Abstract

The invention discloses a method for determining a threshold value of forest and grass vegetation coverage rate that can curb sand production in a watershed. Remote sensing images are used to collect data on the erodible areas of the Loess Plateau completely, and the changes in forest and grass vegetation are combined with data over the years to analyze the impact on the watershed. The influence law of sediment production and the threshold of sediment production were obtained to obtain the threshold of effective forest and grass coverage in different regions and under different rainfall intensities, and then the threshold of forest and grass vegetation coverage that could effectively curb sediment production in the basin was proposed at the basin scale. This method can not only quantitatively describe the sediment reduction benefits of vegetation coverage, but also effectively avoid scale limitations. Based on this idea, it is even possible to gradually extend the research on the watershed scale to the landscape scale, global and regional scales.

Description

A Method for Determining the Threshold of Forest and Grass Vegetation Coverage to Curb Sand Production in Watershed

technical field

The invention belongs to the field of data collection and analysis, and in particular relates to the collection, processing and analysis of rainfall, water and sand, vegetation and terraced field data in the easily eroded area of the Loess Plateau.

Background technique

The Loess Plateau is one of the areas with the most serious soil erosion in my country, and it is also the main source of the Yellow River sediment. Rainfall, soil, topography and vegetation are the key factors that determine the sediment yield of a watershed, and changes in any of these factors may aggravate or slow down soil erosion. At present, it is difficult to change natural factors such as rainfall, macro-topography, and soil composition, but it is possible to affect the sediment yield of the watershed by changing the degree of vegetation coverage and micro-topography. In recent years, the vegetation coverage of the Loess Plateau has been greatly improved, and the sediment entering the Yellow Earth has also been greatly reduced. However, in general, the current research mainly makes a simple qualitative analysis of the relationship between the two, and there are relatively few studies on the quantitative relationship and threshold between the vegetation change of the Loess Plateau and the sediment yield of the watershed; The determination method of the vegetation coverage threshold is mainly based on the observation data of runoff plots. There are relatively few studies on large watersheds or regional scales. In addition, current research techniques continue traditional research methods, and it is impossible to quantify loess The relationship between vegetation changes on the plateau and sediment production in the watershed limits the research on the threshold of vegetation coverage that can curb sediment production.

Therefore, it is urgent to find a new method to obtain relevant data such as vegetation coverage rate and sediment yield index in the erosion-prone areas of the Loess Plateau, so as to understand the influence of vegetation changes on the Loess Plateau on the sediment yield of the watershed, and reveal the quantitative relationship and threshold between the two.

Contents of the invention

The purpose of the present invention is to provide a method for determining the threshold value of forest and grass vegetation coverage that can curb sand production in a watershed, which is suitable for calculating forest and grass vegetation coverage and sediment yield index on a larger watershed scale. High precision.

The technical solution of the present invention is: a method for determining the threshold value of forest and grass vegetation coverage that can curb sand production in a watershed, which is characterized in that remote sensing images are used to collect data that completely covers the erodible areas of the Loess Plateau, and combined with historical data analysis The impact law of forest and grass vegetation changes on the sediment yield of the watershed and the threshold of sediment yield are obtained to obtain the effective coverage rate threshold of forest and grass in different regions and under different rainfall intensities, and then propose forest and grass that can effectively curb the sediment yield of the river basin at the watershed scale. Vegetation coverage threshold. Specifically include the following steps:

(1) Determine the erosion-prone area

Although rainfall, soil, topography, and vegetation are the key factors that determine the sediment yield of the watershed, it is difficult for human beings to change the macro-topography and soil composition of the Loess Plateau. Terraces or horizontal steps, etc.), which in turn affect the sediment yield of the river basin.

A large number of studies have shown that in the urban land of the Loess Plateau, rocky mountainous areas, and river lands and plains with a slope of less than 3° (tableau), by changing human factors such as vegetation coverage and micro-topography, it is found that the sediment yield of the watershed has little effect . Therefore, the river land, plain area, construction land, and hilly land behind the rocky mountain area with a ground slope of less than 3° should be excluded. The other areas are the areas that deserve the most attention in the study of sediment production in the watershed. This paper calls them "erodible areas." .

According to the "Classification of Land Use Status" (GB/T21010-2007), the erodible areas of the Loess Plateau mainly include four types of land use: cultivated land, forest land, grassland and unused land, and the degree of soil erosion depends on the vegetation coverage of forest and grass and the degree of terracing of slope farmland.

(2) Calculation of forest and grass vegetation coverage

Among them, V _c —vegetation coverage of forest grassland; A _is —orthographic projection area of forest grass leaves and stems, unit is Km ² ; A _e —area of erodible area, unit is km ² .

Among them, V _e — forest and grass vegetation coverage rate; A _v — forest and grassland area, unit Km ² ; V _c — forest and grassland vegetation coverage, the ratio of the orthographic projection area of forest and grass leaf stems Al _ls to the forest and grassland area A _v .

(3) Calculation of sediment yield index

Among them, S _i —sediment yield index, refers to the sediment yield per unit area of rainfall in the erodible watershed; W _s —sediment yield of the watershed, the unit is m ³ The sum of the amount of sediment retained by dams and reservoirs, and the amount of sediment diverted by irrigation; P ₂₅ —the total annual rainfall with daily rainfall greater than 25mm, and the total annual rainfall is more sensitive to the sediment yield of the river basin at the basin scale.

(4) Determine the threshold of forest and grass vegetation coverage that can curb sediment production in the watershed

According to the three kinds of rain intensities of 0.075, 0.15 and 0.3, the relationship between vegetation and sediment yield was respectively constructed, and rain intensity = 0.15 and rain intensity = 0.3 were used as the rain intensity benchmarks for determining the Ve threshold, and Si ≤ _7t /km ² ·mm As a sign of basically curbing sediment production in the watershed, the corresponding effective forest and grass coverage rate V _e is the threshold value of _Ve that can basically curb the sediment yield in the watershed.

The present invention aims at the shortcomings of manual data collection that cannot be widely covered and takes a long time, and collects data that completely covers the erodible area of the Loess Plateau through remote sensing images, and analyzes the influence of forest and grass vegetation changes on the sediment yield of the watershed by combining the data of the past years and obtains The threshold of sediment yield is used to obtain the effective coverage threshold of forest and grass in different regions and under different rainfall intensities. Compared with the prior art, the present invention has the following advantages

(1) The spatial resolution of remote sensing images is high, and the data acquisition time is short, which can make up for the lack of human investigation. Compared with the existing research technology, it has a large coverage area, a wide range, comprehensive information, high authenticity, and is easy to operate. It can fully cover the research area—the erosive area of the Loess Plateau, and better reflect the actual coverage of local forest and grass vegetation. .

(2) The research scale is not limited to the slope scale, and can be extended from the watershed scale to the landscape scale, global and regional scales. Quantitatively analyze the influence of forest and grass vegetation changes on the sediment yield of the watershed and the vegetation threshold that can curb the sediment yield of the watershed from a larger scale, which will provide a basis for the later research on reducing sediment into the Yellow River, water and soil erosion control, evaluation, prediction and control Sediment yield in river basins provides a solid theoretical foundation and practical guiding significance.

(3) The calculation method is simple and the calculation accuracy is high.

Description of drawings

Figure 1(a)(b)(c) shows the relationship between the effective coverage rate of forest and grass and the index of sediment yield under the three rainfall intensities of the first to third sub-regions of loess hills on the early underlying surface.

Figure 2(a)(b)(c) is the verification of the relationship between the effective coverage rate and sediment yield of forest and grass terraces under three rainfall intensities in the first to fourth sub-regions of the hills.

Figure 3(a)(b)(c) shows the relationship between effective coverage rate and sediment yield index of forest and grass terraces under three rainfall intensities in the Loess Plateau region and Qiu 5 sub-region respectively.

Figure 4(a)(b)(c) shows the relationship between the effective coverage rate of forest and grass and the index of sediment yield in the arsenic sandstone area, sand-covered area, gravel hill area and loess area, respectively.

Figure 5(a)(b) shows the impact of watershed area change on the response relationship between forest and grass effective coverage and sediment yield index.

Detailed ways

The technical solutions of the present invention will be further described below in conjunction with examples. The invention relates to a threshold method of forest and grass vegetation coverage that can effectively curb sand production in a river basin, can quantitatively describe the sand reduction benefits of vegetation coverage, and effectively avoid scale limitations.

The present invention uses remote sensing images to obtain data such as forest and grassland, combines natural rainfall data to obtain forest and grass coverage and sediment yield index, and analyzes the influence of forest and grass vegetation changes on river basin sediment yield under different landforms, spatial scales, and rainfall conditions on the Loess Plateau, and then At the watershed scale, the threshold of forest and grass vegetation coverage that can effectively curb the sediment production in the watershed is proposed. This method can not only quantitatively describe the sediment reduction benefits of vegetation coverage, but also effectively avoid scale limitations. Based on this idea, the watershed can even be The research on different scales is gradually extended to the landscape scale, global and regional scales.

Among them, the satellite remote sensing images with a spatial resolution of 30m are mainly used to extract the erodible area of each tributary (area), and combined with the obtained data such as forest grass, terraced fields, rainfall, and sand production, to analyze the different landforms, spatial scales, and rainfall conditions of the Loess Plateau. The influence law and threshold value of the change of undergrowth and grass vegetation on the sediment yield of the watershed were analyzed.

The landform types of the Loess Plateau are complex, including the loess hilly and gully area (referred to as the yellow hill area), the loess plateau and gully area (referred to as the plateau area), the loess hilly forest area, the loess terrace area, the highland grassland area, the alluvial plain area, the rocky mountainous area, Arid grassland area, windy sand area, sand-covered hill area, arsenic sandstone area, gravel hill area and loess remnant plateau area. The above classification is divided into loess area and non-loess area according to whether there is loess coverage. Loess hilly gully area, loess plateau gully area, loess hilly forest area, loess step area and highland grassland area belong to the loess area, highland grassland area, and alluvial plain area , earth-rock mountainous area, arid grassland area, windy sand area, sand-covered hill area, arsenic sandstone area, gravel hill area and loess remnant plateau area. Due to differences in topography, landform and erosion intensity, the Huangqiu area is divided into five sub-areas (hereinafter referred to as Qiu 1 sub-area, Qiu 2 sub-area, Qiu 3 sub-area, Qiu 4 sub-area, and Qiu 5 sub-area). 90% of the sediment in the Yellow River comes from the Huangqiu area and the plateau area, so focus on these two areas.

In the study area, a curve is simulated according to the relationship between the three kinds of rainfall intensities on the effective coverage rate of forest and grass and the sediment yield index to analyze the relationship between the effective coverage rate of forest and grass and the sediment yield index, and finally obtain the threshold value.

1. Research Methods

First of all, after defining the acquisition method of forest and grass data, new concepts such as area of erodible area, effective coverage rate of forest and grass, and sediment yield index are introduced, so that tributaries with different drainage areas and different rainfall conditions have a unified evaluation of sediment yield Based on the standards, the following methods were used to carry out the research:

1. According to the soil type and terrain characteristics, select a watershed with a medium watershed area and very few terraces as a sample watershed, and directly establish a response relationship between the measured sediment yield index before 1999 and the effective coverage rate of forest and grass in the same period, and analyze the change law.

2. In view of the fact that the types of forest and grass vegetation in the main sand-producing areas of the Yellow River, the protection methods of leaves, stems and roots on the surface are different from those before the 1990s, the measured data of the sample watershed in the past ten years were collected to verify the response relationship or fix.

3. Define the scientific connotation of "containment of sediment production in the river basin", and propose the effective coverage rate threshold of forest and grass that can effectively curb the sediment production in the river basin.

Rain intensity is obviously a key factor affecting sediment yield. According to the actual situation of the Loess Plateau, the relationship between vegetation and sediment yield was constructed respectively under three rainfall intensities of 0.075, 0.15 and 0.3.

2. Threshold of effective forest and grass coverage that can effectively curb sand production

It can be seen from Figure 1(a)(b)(c) and Figure 2(a)(b)(c) that when the effective coverage of forest and grass is ≥60%, although the average value of Si is 4.4t/km ² ·mm, However, there are a few data of Si reaching 10-15t/km ² ·mm. Therefore, it is necessary to define the connotation of "containment of sediment production in the river basin".

According to the "Soil Erosion Classification and Grading Standard" (SL190-2007), from the safety point of view, "S _i ≤ 7t/km ² ·mm" is used as a sign to basically curb the sediment production in the watershed, and the corresponding effective forest and grass coverage rate V _e is is the _Ve threshold that can basically curb the sediment production in the watershed. From the perspective of partial safety, for the low rain intensity area and the high rain intensity area, "rain intensity = 0.15" and "rain intensity = 0.3" are used as the rain intensity benchmarks for determining the Ve threshold.

Strictly follow the principle of "the proportion of terraced fields is less than 3%", select 85 pairs, 124 pairs and 62 pairs respectively from the data in Figure 2(a)(b)(c) and Figure 3(a)(b)(c) Based on the data (the effective coverage rate V _e of forest and grass ranges from 5.7-95%, and the proportion of terraced fields has been included in V _e in equal amounts), the relationship lines of V _e -S _i under three kinds of rain intensities are plotted, and the first The quantitative response relationship between the effective coverage rate of forest and grass and the sediment yield index in the -4 sub-region under the three rainfall intensities is as follows: R ² is the correlation coefficient, and the larger the correlation, the better the correlation.

S _i =365e ^-0.078Ve R ² =0.75 (Formula 1)

S _i =418e ^-0.076Ve R ² =0.80 (Formula 2)

S _i =540e ^-0.074Ve R ² =0.69 (Formula 3)

(1) Figure 1(a)(b)(c) respectively shows the response relationship between forest and grass effective coverage rate and sediment yield index in the 1st to 3rd sub-regions of Huangqiu under three rainfall intensities, and the data used are all from 1966 to 1999 year data. It can be seen from Figure 1(a)(b)(c) that although the topography of the three sub-regions is different, there is no difference in the sediment yield index under the same forest and grass coverage, that is, the sediment yield per unit area under the same rainfall The quantity is basically the same.

(2) With the regression line in Figure 1(a)(b)(c) as the background, distinguish between two situations where the proportion of terraced fields is less than 6% and greater than 15%, and plot the measured data from 2009 to 2018 in Figure 2 ( In a) (b) (c) (the proportion of terraced fields is included in the effective coverage of forest and grass), the results show that: regardless of the intensity of rainfall, the effective coverage of forest, grass and terraced fields in the past ten years has a significant relationship with the sediment yield index of the watershed. The relationship still obeys the previous law.

Based on the above three formulas, according to the standard of "sediment yield index ≤ 7t/km ² ·mm", for the low rain intensity area and high rain intensity area of the 1st to 4th sub-areas of hills, the Ve thresholds are 55% and 60% respectively. For the Loess Plateau with harsh natural conditions, "sediment yield modulus ≤ 1000t/km ² ·a" is an extremely high standard. Therefore, the sediment yield modulus should be controlled below 2500t/km ² ·a (corresponding sediment yield index≤17.5t/km ² ·mm). To achieve this goal, for the low rain intensity area and high rain intensity area of the 1st to 4th sub-regions of hills, the forest and grass threshold Ve is 42% and 47%, and 42% to 47% is just at the inflection point of the relationship line, as shown in Figure 1 ( a)(b)(c) and Figure 2(a)(b)(c).

(3) For the Qiu 5 subregion and the loess plateau region, when the effective coverage rate of forest and grass terraces is greater than 45-50%, the sediment yield of the watershed tends to be stable. However, due to the special mechanism of sediment production, even if the effective coverage rate of forest grass and terraced fields reaches 70%, it is difficult to curb the sediment production.

(4) Taking the response relationship (Equation 2) of sub-regions 1 to 4 as the background, Fig. 4(a)(b)(c) respectively shows the response relationship of the pistyl sandstone region, sand-covered region, gravel hilly region and loess region Relationship between forest and grass ~ sand production. It was found that, under the same vegetation and rainfall conditions, the sandstone area had the highest sand production, followed by the loess area, and the sand-covered area and gravel hills area had the least sand production. The threshold value of Ve is 45-50% in the sand-covered area and the gravel hill area; 70-80% in the arsenic sandstone area.

(5) The sample watershed area in Figure 1(a)(b)(c) to Figure 3(a)(b)(c) varies from 500 to 5000 km ² , so it is necessary to demonstrate the impact of spatial scale changes on forestry grass-sediment production relationship impact. Among the data with rainfall intensity of 0.075 and 0.15, two groups of data with large difference in watershed area were selected for redrawing, as shown in Figure 5(a)(b). The effect of the sand index relationship is not obvious.

The above examples are only used to illustrate the technical solutions of the present invention. The present invention is not limited to the above-mentioned embodiments. Any modifications, equivalent replacements, improvements, etc. within the spirit and principles of the present invention should be included in this document. within the scope of protection of the invention.

Claims (1)

1. A method for determining a forest and grass vegetation coverage rate threshold value capable of restraining sand production in a river basin is characterized in that a remote sensing image is used for carrying out data acquisition for completely covering an easily-eroded area of a loess plateau, influence rules of forest and grass vegetation change on the sand production in the river basin are analyzed by combining historical data, a sand production amount threshold value is obtained, effective forest and grass coverage rate threshold values in different areas and under different rain intensities are obtained, and then the forest and grass vegetation coverage rate threshold value capable of effectively restraining the sand production in the river basin is provided on the scale of the river basin;

the method specifically comprises the following steps:

(1) Determining erodible zones

The method is characterized in that other areas of the land in the hilly area after removing river lands, plain areas, construction lands and rocky mountain areas with the ground gradient less than 3 degrees are also the areas most concerned in the research of the sand production in the watershed;

(2) Calculating the vegetation coverage rate of the forest and grass

Wherein, V _e -forest and grass vegetation coverage; a. The _v -area of forest and grass in Km ² ；V _c Vegetation coverage of forest grassland, orthographic projection area A of forest grass leaves and stems _ls Area of forest lawn A _v The ratio of (a); a. The _is -area of orthographic projection of the leaves and stems of forest grass in Km ² ；A _e Area of erodible zone in km ² ；

(3) Calculating sand production index

Wherein S is _i -sand production index, which refers to the sand production per unit area per unit rainfall in the easily erodable watershed; w _s -sand production in river basin in m ³ The sum of the sand conveying amount, the sand blocking amount of the check dam and the reservoir and the irrigation sand guiding amount is actually measured on the section of the opening in the formula; p is ₂₅ -total annual rainfall of more than 25mm daily, which is more sensitive to watershed sand production on a watershed scale;

(4) Determining forest and grass vegetation coverage rate threshold capable of restraining sand production in drainage area

Respectively constructing the relationship between vegetation and produced sand according to three rain intensity conditions of 0.075, 0.15 and 0.3, taking rain intensity =0.15 and rain intensity =0.3 as rain intensity references for determining a Ve threshold value, and taking S as a reference for determining the Ve threshold value _i ≤7t/km ² Mm is used as a mark for basically restraining sand production in the drainage area, and the corresponding effective coverage rate V of the forest and grass _e I.e. V capable of basically restraining sand production in the drainage area _e A threshold value.

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