Table 1 Evaluation index system and grade criteria of land degradation in Bijiang watershed

Slight degradation

Moderate

degradation

Severe

degradation

Extremely-severe degradation

Water-soil erosion

Precipitation

<800

800 -1000

1000-1200

1200-1500

intensity

soil texture

Coarse sand, clay soil

loamy soil

sandy loam soil, silt clay soil, loamy clay soil

sandy mealy soil, mealy soil

Terrain slope

<15

15-25

25-35

35-90

NDVI[16]

[0.2,1)

[-0.15,0.2)

[-0.35,-0.15)

[-1,-0.35)

The risk of geological disaster

Geological disaster density [17] (points/km²)

<1

1-3

3-5

>5

hypsography degree [18]

<250

250-500

500-800

>800

Occurrence level of geological disaster

Easy occurrence area

Moderately easy occurrence area

Highly easy occurrence area

Excessively easy occurrence area

ratio of land with a slope above 25°

<8

8-30

30-60

>60

Biodi-

versity

SIEI

>1.3

1.2-1.3

1.0-1.2

<1.0

Important species

First-grade State protection species

Second-grade State protection species

Other State protection and provincial protection species

Other regional protection species

Index of biological abundance [19]

>70

55-70

40-55

<40

Cultiv-

ation intensity

Cultivation index of arable land

<0.35

0.35-0.6

0.6-0.8

0.8-1

ratio of arable land with a slope above 25°

<0.02

0.02-0.03

0.03-0.05

>0.05

Pollution of heavy metals in

Content of Cd in soil [20]

<0.2

0.2-0.35

0.35-0.6

>0.6

soil

Content of Zn in soil [20]

<250

250-300

300-500

>500

Content of Pb in soil [20]

<100

100-300

300-450

>450

Content of As in soil [20]

<15

15-25

25-30

>30

Assigned value

3

5

7

9

Grade standard for value

2.1-4.0

4.1-6.0

6.1-8.0

>8.0