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Revision as of 14:53, 19 September 2012

The tutorial is prepared to use the J2000 hydrological model for hydrological system analysis of a river catchment. A test catchment and dataset of the Dudh Kosi river basin has been provided along with the tutorial. The Dudh Kosi river basin was used for the hydrological system analysis by using the J2000 hydrological model as a part of the PhD research (Nepal, 2012). The information provided here is largely based on this study. PhD Thesis. The motivation, objectives and methodological apporach and the rational of using the J2000 model in the Dudh Kosi river basin are also presented. The users can use the test data to get familiar with the model application. At the same time, users can prepare their own dataset to understand the hydrological system dynamics of any river basin by following this tutorial.

Precipitation distribution module

This tutorial describe the important processes and algorithm of the different modules within the J2000 hydrological model:

Calibration parameters

parameter	description	Global range	For Dudh Kosi model
Trans	threshold temperature	0 + 5	2
Trs	base temperature for snow and rain	-5 +5	0

In the J2000 modelling system, the precipitation is first distributed between rain and snow depending upon the air temperature. Two calibration parameters (Trans, and Trs) are used where Trs is base temperature and Trans is a temperature range (upper and lower boundary) above and below the base temperature. In order to determine the amount snow and rain, it is assumed that precipitation below a certain threshold temperatures results in total snow precipitation and exceeding a second threshold results in total rainfall as precipitation. In the range (Trans) between those threshold temperatures, mixed precipitation occurs. Between those thresholds, rain-snow mixtures with variable percentages for each component are calculated. The acutal amount of snow (P(s)) of daily precipitation subject to air temperature is calculated according to:

$Ps = \frac{TRS + Trans - T}{2 \cdot Trans} \, \, \, \mathrm{[mm]}$

The daily amount of snow (P_s) or amount of rain (P_r) is calcualted according to:

$Ps = Precipitation \cdot Ps \,\,\, \mathrm{[mm]}$

$Pr = Precipitation \cdot (1- Ps) \,\,\, \mathrm{[mm]}$

These parameters are considered as non-flexible parameters and not necessarily placed in the JAMS framework as tunable parameters.

Relevancies in modelling

Putting the Trs values below zero (e.g. 2) will bring more precipitation in the form of 'rain' than 'snow'.

Interception module

Interception is a process during which the precipitation is stored in leaves, and other open surfaces of vegetation. During precipitation, interception by crop canopy and residue layer occurs. This process is identified as important components of a hydrological cycle that can affect the water balance components. Canopy and residue interception are considered losses to the system, as any rainfall intercepted by either of these components will subsequently be evaporated (Kozak et al. 2007). The interception module in the J2000 modelling system serves the calculation of the net precipitation from the observed precipitation against the particular vegetation covers and its development in the annual cycle. The observed precipitation is reduced by the interception part to calculate the net precipitation. Thus net precipitation only occurs when the maximum interception storage capacity of the vegetation is reached. The surplus is then passed on as throughfall precipitation to the next module. The interception module uses a simple storage approach according to Dickinson (1984), which calculates a maximum interception storage capacity based on the Leaf Area Index (LAI) of the particular type of land cover. The emptying of the interception storage is done exclusively by evapotranspiration. The maximum interception capacity (Int_max) is calculated according to the following formula:

$Int_{max} = \alpha \cdot{LAI} \, \, \, \mathrm{[mm]}$

with

α ... storage capacity per m² leaf area against the precipitation type [mm]

LAI ... LAI of the particular land-use class provided in land-use parameter file [-]

The parameter a has a different value, depending on the type of the intercepted precipitation (rain or snow), because the maximum interception capacity of snow is noticeably higher than of liquid precipitation. The LAI for individual vegetation types is provided in the land-use parameter file throughout the year. Because the LAI changes according to the seasons, four different LAI types for four different seasons for each vegetation type are proposed in land-use parameter file. The value of LAI can be determined by direct measurement of leaves, literature, and expert knowledge.

Calibration parameters

parameter	description	Global range	For Dudh Kosi model
α_rain	storage capacity (m²) of particular land cover for rain	0 to +5	1.0
α_snow	storage capacity (m²) of particular land cover for snow	0 to +5	1.28

Relevancies in modelling

Keeping the high value of α will store more water on leave surfaces which leads to higher evapotranspiration and less water available for runoff and vice versa.

Snow module

Calibration parameters

parameter	description	Global range	For Dudh Kosi model
snowCritDens	Critical density of snow	0 to 1	0.381
snowColdContent	cold content of snowpack	0 to 1	0.0012
baseTemp	threshold temperature for snowmelt	-5 to 5	0
t_factor	melt factor by sensible heat	0 to 5	2.84
r_factor	melt factor by liquid precipitation	0 to 5	0.21
g_factor	melt factor by soil heat flow	0 to 5	3.73

These parameter are provided in bold and italic letters in the description below:

The snow module calculates the different phases of snow accumulation, metamorphosis and snowmelt. The more complex module is adapted in the model from Knauf (1980). The snow module takes into account the changes of state of snow pack during its existence, especially changes of snow density due to melting and subsidence. This process is important because snow pack can store free water, like a sponge, until reaching a certain threshold density and only then a sudden discharge of water occurs. For the model different water capacities of the snow pack are considered: the actual snow water equivalent (SWE_dry) which corresponds to the amount of water which has actually frozen and the total snow water equivalent (SWE_total) which in addition considers liquid water stored in the snow pack. The subsidence of the snow pack, which results from the liquid water through the snowmelt to the surface or from precipitation as rainfall, is calculated according to the empirical subsidence (snow-compaction scheme) by Bertle (1966).

The snow pack and its conditions are described in the modell according to the following parameters: snow depth (SD)[mm], dry snow density (dryDens)} [in g/cm³] as the quotient from total water content and snow depth.

If there is minimum, mean or maximum air temperature for a certain time (daily data), the module calculates separate accumulation or melt temperatures. Accumulation and melting can occur within a time step. The accumulation and melt temperatures (T_acc and T_melt) can be calculated according to:

$T_{acc} = \frac{T_{min} + T_{avg}}{2} \,\,\,\,\,\,[^oC]$

$T_{melt} = \frac{T_{max} + T_{avg}}{2} \,\,\,\,\,\,[^oC]$

Accumulation phase:

The snow module simulates accumulation and compaction of the snow pack caused by snowmelt or rain on snow precipitation.

The thermal circumstances under the snow cover are taken into account with the cold content in the snow cover in connection with the snowmelt. At the temperature below the freezing point, the snow pack cools down significantly. Because melted water freezes immediately due to negative isothermal circumstances under the snow cover, no runoff occurs. The cold content needs to reach the value zero so that the process of snowmelt begins again. Consequently, negative temperatures raise the cold content whereas the positive temperature reduces it. The calculation of storage of cold content results from the product of air temperature by a calibration parameter (coldContFact).

$CC = coldContFact \cdot T \,\,\,\,\,\,[mm]$

In doing so, negative air temperatures are accumulated and decreased only by positive temperature and resulting potential rates of melting. Only when the cold content has reached a value of 0, snowmelt occurs.

If the air temperature is below -15 C, the density of the new snow is assumed to be 0.02875.

The change of snow depth (δ SD) resulting from snow precipitation is calculated according to : Snow accumulation occurs in the model if precipitation falls in solid form (newSnow > 0). Therefore the density of new snow is determined subject to air temperature. The calculation is carried out according to (Kuchment 1983, and Vehvilaeinen 1992), if the air temperature is higher than -15 oC.

$newSnowDens = 0.13 + 0.0135 \cdot T_{acc} + 0.000045 \cdot T^{2}_{acc}\,\,\,\, [g/cm^3]$

If the air temperature is below -15 oC, the density of the new snow is assumed to be 0.02875.

The change of snow depth (δ SD) resulting from snow precipitation is calculated according to :

$δ SD = \frac{netSnow}{newSnowDens} \,\,\,\,\,\,[mm]$

The snow water equivalent of the previous day (\textit{SWEdry}) increases by the value of snow precipitation according to:

$SWEdry_{{t}} = SWEdry_{{t-1}} + netSnow \,\,\,\,\,\,[mm]$

The dry snow water equivalent and the total snow water equivalent are increased by the same value. If the precipitation event involved mixed (rain/snow) precipitation, the rain amount is allocated to the total snow water equivalent.

If rain is part of the precipitation event, it results in subsidence of the snow pack. The calculation of the subsidence amount is discussed below. In the model, the snow pack remains in the accumulation phase until the temperature value (T_melt) for the snowmelt exceeds a threshold value (baseTemp)which has to be determined during the parameterisation phase of the modeling application. Then it enters the metamorphosis phase which simulates melting and subsidence processes. However, it can go back to the accumulation phase if temperatures are correspondingly low. Due to different temperature values, accumulation and melting processes can be modeled during one time step.

Melting and subsidence phase:

If the melt temperature value (T_melt) exceeds the temperature limit value (baseTemp), the snow pack goes from the accumulation phase to the metamorphosis. The amount of energy which is required for snowmelt is available in three different ways. First, by input of sensible heat by air temperature (t_factor), second, by energy input from precipitation as rain (r_factor) and third, by input due to soil heat flow (g_factor). The sum of all energy inputs gives the potential snowmelt rate (Mp). The calculation of Mp is carried out according to:

$Mp = t\_factor \cdot T_{melt} + r\_{factor} \cdot netRain \cdot T_{melt} + g\_{factor} \,\,\,\,\,\,[mm]$

The variable Mp is then also modified according to the slope and the exposition of the spatial model entity (i.e. HRU):

$Mp = \ Mp \cdot actSlAsCf \,\,\,\,\,\,[mm]$

Mp is initially used to balance out the cold content of the snow cover and is then also used to generate snowmelt. The potential snowmelt rate then is taken to calculate the resulting maximum change of snow depth (δ SD):

$δ SD = \frac{M_P}{dryDens} \,\,\,\,\,\,[mm]$

If δ SD is greater than the entire snow depth, it defrosts completely and the entire snow water equivalent contributes to runoff generation in the form of snowmelt. If this is not the case, the snow depth is reduced correspondingly, which does not change the snow water equivalent at first. Rather the result is an increase in the total density of the snow cover.

In addition to this change in density, additional changes in subsidence and density according to the snow compaction-scheme (Bertle 1966) are taken into account. This method is based on the fact that water, no matter whether it results from temperature-induced snowmelt or from precipitation, seeps into the snow pack which leads to subsidence by recrystallization of snow and by structural changes and concentration in the storage (Knauf 1980). The resulting subsidence rate is calculated using the snow-subsidence method described in Bertle (1966). This method is based on the observation of an empirical relation between inflowing free water and the resulting change in elevation by subsidence which was derived from laboratory experiments of the US Bureau of Reclamation. For the calculation the increase of accumulated water content in percentage is seen in relation to the snow water equivalent using this formula:

$P_w = \frac{totSWE}{drySWE} \cdot 100 \,\,\,[\%]$

This equation shows that the more liquid water there is as input, the greater is the snow pack subsidence (P\_w) (Knauf 1980). An input of the exact the amount of water corresponding to the snow water equivalent of the snow pack leads to halving the snow depth by subsidence. The percentage of snow depth change (P$_H$) is calculated subject to the input of free water:

$P_H = 147.4 - 0.474 \cdot P_W \,\,\,[\%]$

The new snow depth (SD) is:

$SD = SD \cdot \frac{P_H}{100} \,\,\,[mm]$

Together with the snow depth which has been calculated the total density \textit{(totDens)} and the dry snow density \textit{(dryDens)} are calculated according to the following formulas:

$dryDens = \frac{SWE_{dry}}{SH} \,\,\,[g/cm^3]$

$totDens = \frac{SWE_{tot}}{SH} \,\,\,[g/cm^3]$

Melt runoff

The snow pack can store liquid water in its pores up to a certain critical density (snowCritDens). This storage capacity is lost nearly completely and irreversibly when a certain amount of liquid water in relation to the total SWE (between 40 and 45 percent) is reached according to Bertle (1966), Herrmann (1976) and Lang (2005). In this threshold limit, the retention capacity of a naturally developing snow pack is also suddenly decreased without rain impact. In such a case, a sudden water release from the snow pack can be observed (Herrmann 1976). In the model, this process is simulated by using the calculation of a maximum water content of the snow pack (SWE_max) according to :

$WS_{max} = snowCritDens \cdot SD \,\,\,\,\,\,[mm]$

The critical density (snowCritDens) needs to be provided by the model user. The water stored in the snow pack which exceeds this limit is conveyed as snow runoff (Q_snow).

$Q_{snow} = SWE_{tot} - SWE_{max} \,\,\,\,\,\,[mm]$

In the following time steps, the density of the snow pack keeps the critical threshold density until it is either defrosted or starts the accumulation due to recurring snowfall.

Relevancies of calibration parameter with model results:

baseTemp is a threshold temperature for snow melt. The melting only occurs if the surrounding temperature is higher than baseTemp. Keeping the value high will make more snow to store and less snowmelt occurs and vice versa. This parameter is very important in case the basin has seasonal snow storage and melt. The best value can be found around the value 0.

snowCritDens is an important calibration parameter as this allows liquid water from snowpack to be melt when the critical density is higher than this value. This storage capability is lost nearly completely and irreversibly when a certain amount of liquid water proportionally to the total snow water equivalent (around 40%) is reached. Keeping this parametr value low will release the liquid water from snowpack quickly (asthe critical density of snow is reached faster ) with less amount of snow and related depth. The high value results more time to reach the critical desnity which will require more fresh snow to occur.

snowColdContent helps to reach the cold content of a snowpack close to zero so that the melting process start. Higher value will help to reach the cold content of a snowpack to zero faster than low value.

t_factor is one of the most important and sensitive parameters to produce melt runoff from a snowpack. The higher value will produce higher snowmelt and vice versa for low value. The higher t_factor value along with r_factor and g_factor will provide heat energy to melt the snowpack and produce runoff.

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 This tutorial describe the important processes and algorithm of the different modules within the J2000 hydrological model:
-= Precipitation distribution module =
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 |-
 |}
-= Precipitation distribution module =
 In the J2000 modelling system, the precipitation is first distributed between rain and snow depending upon the air temperature. Two calibration parameters (''Trans'', and ''Trs'') are used where ''Trs'' is base temperature and ''Trans'' is a temperature range (upper and lower boundary) above and below the base temperature. In order to determine the amount snow and rain, it is assumed that precipitation below a certain threshold temperatures results in total snow precipitation and exceeding a second threshold results in total rainfall as precipitation. In the range (''Trans'') between those threshold temperatures, mixed precipitation occurs.  Between those thresholds, rain-snow mixtures with variable percentages for each component are calculated. The acutal amount of snow (P(s)) of daily precipitation subject to air temperature is calculated according to:

J2000 modules in detail

Revision as of 14:53, 19 September 2012

Precipitation distribution module

Interception module

Snow module

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