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Chapter 6
Forecasting Models
A Forecasting model is the kernel of a forecasting system.
In order to meet the requirement of
standardization of forecasting model, standardization of input and output will be made for the common
used conceptualized models, so to establish standardized forecasting model library in China. This chapter
will introduce 5 common used models in China, including Three-Layer Soil Moisture Model (SMS_3),
Lag-and-Route Method (LAG_3), MUSKINGUM Routing Method (MSK), Rainfall-Runoff Correlation
Diagram Model (P_RZHJR) and Empirical Unit Hydrograph (UH_B), describe the interfaces of various
input files, and offer the usual methods of debugging the model parameters.
6.1 Three-Layer Soil Moisture Model (SMS_3)
6.1.1 Model Principle
Three-Layer Soil Moisture Model (SMS_3) includes 3 kinds of calculation of evapotranspiration,
runoff yield and water sources division.
1. Calculation of evapotranspiration
Basin evapotranspiration is calculated with three-layer mode. The equations are as follows:
E p = K × E0
（12.1）
in which, Ep means evapotranspiration ability; E0 means observed evaporation; and K means reduction
coefficient of evaporation.

Ep≥
当P + WU ≥ E p时


WL
WL
E =  E p − WU − P
当P + WU < E p 且
> C时
WLM
WLM

 C E − WU − P ≤ 当P + WU < E 且 WL ≤ C时
p
p

WLM
(
)
(
（12.2）
)
in which, C means reduction coefficient of deep layer evaporation; WU、WL means soil moisture content in
upper and lower layers; WLM means lower layer tension water capacity; P means precipitation; and E
means calculated evaporation.
2. Calculation of runoff yield
It is considered that soil moisture deficit is equal to water storage capacity on a basin according to a
basin water storage capacity curve. Suppose water storage capacity as Wm, of which the maximum is
Wmm, and suppose that the basin water storage capacity curve is a b times parabola, so that the curve can
be expressed as follows:
f
Wm 

= 1 − 1 −

F
 Wmm 
b
（12.3）
Hereby, mean water storage capacity on a basin (WM) can be obtained as
WM =
Wmm
1+ b
（12.4）
８９