GB/T 20486-2026Grades of watershed areal rainfall (English PDF)

Foreword

This document complies with the provisions of GB/T 1.1-2020 "Standardization Work Guidelines Part

1.Structure and Drafting Rules of Standardization Documents". Drafting. This document replaces GB/T 20486-2017 "Rainfall Grades in River Basins". Compared with GB/T 20486-2017, the main differences are structural adjustments. Aside from editorial changes, the main technical changes are as follows:

a) The definition and English equivalent of "watershed" have been changed (see 3.1,

2.1 in the.2017 edition);

b) The definition and English equivalent of station rainfall have been changed (see 3.2,

2.2 in the.2017 version);

c) Added terminology and definitions for gridded rainfall (see 3.3);

d) The terms and definitions for rainfall levels have been removed (see section

2.3 of the.2017 edition);

e) The definition and English equivalent of areal rainfall have been changed (see 3.4,

2.4 in the.2017 edition);

f) The threshold ranges for 3-hour and 6-hour areal rainfall levels have been expanded (see Table 1 in Chapter 4);

g) The method for calculating areal rainfall has been changed and its scope of application has been expanded (see Chapter 5, Chapter 4 of the.2017 edition). Please note that some content in this document may involve patents. The issuing organization of this document assumes no responsibility for identifying patents. This document was proposed by the China Meteorological Administration. This document is under the jurisdiction of the National Technical Committee on Standardization of Meteorological Disaster Prevention and Mitigation (SAC/TC345). This document was drafted by the National Meteorological Center, Henan Provincial Meteorological Observatory, Information Center of the Ministry of Water Resources, and Anhui Provincial Meteorological Information Center. The main drafters of this document are. Xu Fengwen, Wang Meng, Bao Hongjun, Wang Li, Cao Shuang, Di Jingyue, Huang Changxing, Ye Jinyin, Wang Di, and Li Yumei. Yun Xiaobo and Yang Yin. The release history of this document and the document it replaces is as follows:

---First published in.2006 as GB/T 20486-2006, and revised for the first time in.2017;

1 Scope

GB/T 20486-2026 is the Chinese national standard covering rainfall averaged over a river basin rather than measured at a point - the quantity that actually determines whether a river will rise, and the grades into which it is classified for warning purposes. It replaces GB/T 20486-2017 and has been in force since 1 July 2026. It was issued on 31 March 2026 and has been in force since 1 July 2026, replacing GB/T 20486-2017. The document is under the responsibility of the China Meteorological Administration. This page is published from the official record of the 2026 edition; the clause text of a standard this recent is not yet in circulation, and the figures, limits and tables it contains are those of the document itself, delivered in full with the English translation.

This document specifies the classification and calculation methods for areal rainfall levels in river basins. This document applies to operational and scientific research activities related to the monitoring, forecasting, and service of surface rainfall in river basins.

4.Classification of Aspect Rainfall Levels Based on the amount of areal rainfall at different times, the areal rainfall in river basins is divided into light rain, moderate rain, heavy rain, rainstorm, torrential rain, and extremely heavy rain. There are 6 levels. The specific classification should conform to the provisions of Table 1.

5 Methods for calculating surface rainfall

5.1 The arithmetic mean method, Thiessen polygon method and Kriging method are commonly used for the calculation of areal rainfall, as detailed in Appendix A.

5.2 When the watershed area is small, the terrain is relatively flat, and there are many rain gauges evenly distributed, the arithmetic mean method should be used; based on gridded rainfall... This method is also suitable for data calculation, see A.1.

5.3 When rain gauges are unevenly distributed, the Thiessen polygon method should be used, see A.2.

5.4 When the topography of the watershed is complex and the spatial distribution of rainfall is uneven, the Kriging method should be used, see A.3. GB/T 20486-2026. Rainfall Classification in River Basins ICS

07.060 CCSA

47 National Standards of the People's Republic of China Replaces GB/T 20486-2017 Rainfall levels in river basins Published on 2026-03-

31 Implemented on July 1, 2026 State Administration for Market Regulation The State Administration for Standardization issued a statement.