Drought Early Warning System

Using some of the indices described above, an operational system for drought early warning for North China Plain (NCP) was designed (Tian et al., 1997), as illustrated in figure 28.3. Atmospheric and geometric corrections are made in the AVHRR data before estimating temperature and albedo. Meteorological data and CWSI images are appropriately registered. Finally, the system integrates drought models and remote sensing and meteorological data to produce a series of drought maps and reports.

Geographic Information System Database

The Geographic Information System (GIS) database has been set up for the management and application of historical data and real-time meteorological data for drought monitoring in NCP. The database comprises: (1) thematic, county-level, soil texture, water table, and geomorphological maps; (2) meteorological data, including rainfall data (1949-94), daily meteoro logical data from February to June (1992-97), and dekadal (10-day) data for agricultural season (1992-97); (3) experimental data at Yucheng Ecosystem Experimental Station, Chinese Academy of Sciences (CAS), in Shandong province, and Fenqiu Ecosystem Experimental Station and CAS in Henan province (1992-93); (4) socioeconomical data including crop productivity, population, and county area; (5) AVHRR data (1992-97); and (6) maps showing drought distribution and severity. The GIS integrates these different data sets (remote sensing data, CWSI, and NDTI models) to map soil moisture, evapotranspiration, and, finally, drought conditions.

Timeslice Products

Tian (1993) monitored droughts in the NCP region by estimating CWSI and relating it to soil moisture as follows:

Wf where W is the soil moisture, Wf is the field capacity, and R is the correlation coefficient. Table 28.3 indicates the accuracy of drought monitoring with the CWSI model. Figure 28.4 shows how timeslice products are used to monitor drought in NCP.

Soil Humidity Index The SHI is calculated as

where SHI, is the ratio of soil moisture (Q,) to field capacity (SW,), and j varies from 1 to 10 for soil measurement. The relationship between the vegetation condition index (VCI) and SHI was established to enable the use of the AVHRR data to monitor droughts in China. The soil moisture

Table 28.3 Accuracy of drought monitoring using crop water stress index model in 1994

Table 28.3 Accuracy of drought monitoring using crop water stress index model in 1994

City/province

March 4

March 26

April 13

April 30

May 8

Beijing

84.0

86.0

85.0

88.0

85.0

Tianjin

88.0

88.8

82.5

85.0

87.2

Henan

90.6

87.5

83.1

88.8

86.2

Anhui

83.3

80.6

86.6

83.3

83.3

Jiangsu

80.0

Cloud

75.0

81.5

80.0

Shandong

81.4

85.7

82.8

82.8

85.7

Hebei

85.7

80.0

84.2

87.1

85.7

Figure 28.4 Timeslice products for drought monitoring in the North China Plain.

at 20 cm depth and the corresponding field capacity information from 102 agricultural climate observatories (covering the country) for the period from 1980 to 1994 have been collected. SHI had good correlation with VCI (table 28.4, figure 28.5).

The correlation coefficients ranged from 0.60 to 0.85. The test results of a statistical model of all 36 ten-day periods from June to December indicated that the model of each dekad passed the test at the level of a = 0.05. Figure 28.6 illustrates an operational system of drought early warning. Some results using this system are shown in figures 28.7 and 28.8.

Table 28.4 Statistical model of drought monitoring and associated drought category

Dekad in July Model

Drought categorization R Range of VCI Drought category

Dekad in July Model

Drought categorization R Range of VCI Drought category

<90

Heavy drought

90-109

Dry

1

VCI

= 219.2832 + 83.8763 x

LN(SHI)

0.846

109-124

Light dry

124-148

Normal

>148

Moist

<38

Heavy drought

38-44

Dry

2

VCI

= 20.6373 x exp(0.0152

x SHI)

0.802

44-51

Light dry

51-70

Normal

>70

Moist

<31

Heavy drought

31-41

Dry

3

VCI

= 12.736 x exp(0.0235 x

SHI)

0.856

41-52

Light dry

52-83

Normal

>83

Moist

Figure 28.5 The scatter plot between measured soil humidity index (SHI) and vegetation condition index (VCI).
Figure 28.6 Operational system for drought early warning for China.
Figure 28.7 Drought distribution in China during third dekad (10 days) of May 2000.
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