Start | Index | Overview • PREV | UP | NEXT Snow Cover is calculated using the formula:
DST0=(1.-BCV)*DST+BCV*STMP(2)) [1]
Reference: Williams, J.R., Izaurralde, C.A., 2005. The APEX model, Blackland Research Center Reports, Vol. 2. Blackland Research Center, USDA, Temple, Texas, USA
If the soil surface is not bare, the surface temperature can be affected considerably by the amount of cover (crop residue or snow). This effect can be simulated by combining the estimated bare surface temperature for the day with the previous day's temperature in the second soil layer (the top 10 mm layer is considered too thin for this purpose).
where DST0 is the final estimate of soil surface temperature in °C and BCV is a lagging factor for simulating residue and snow cover effects on surface temperature. The value of BCV is 0.0 for bare soil and approaches 1.0 as cover increases, as expressed in the equation
BCV=max(SNOF,BCV') [2] BCV'=CV/(CV+exp(5.34-2.40*CV)) [3] SNOF=SNO/(SNO+exp(2.30-0.220*SNO)) [4]
where CV is the sum of above ground biomass and crop residue in t ha-1 and SNO is the water content of the snow cover in mm.
If snow is present, it may be melted on days when the second soil layer temperature exceeds 0 oC. Snow is melted as a function of the snow pack temperature using the equation
SML=AMAX1(0.,X1*(1.52+.54*F*SNPKT)); 0.0<SML<SNO [5] SNPKT=.3333*(2.*X2+TX) [6] X1=sqrt(TMX*RA) [7] X2=min(DST0,STMP(2)) [8] F=TSNO/(TSNO+exp(5.34-2.395*TSNO)) [9]
where SML is the snowmelt rate in mm d-1, SNO is the snow present in mm of water, STMP is the temperature in oC of soil layer 2, SNPKT is the snow pack temperature in oC, DST0 is the soil surface temperature in oC, and TSNO is the age of the snow pack in d.
The equations for estimating STMP and DST0 are presented in the soil temperature section. Melted snow is treated the same as rainfall for estimating runoff volume and percolation, but rainfall energy is set to 0.0 and peak runoff rate is estimated by assuming uniformly distributed rainfall for a 24-h duration.
EAJ is a soil cover index. The value of EAJ ranges from 0 to 1.0 according to the equation
EAJ=exp(-X1) [10] X1=max(0.4*SMLA,0.1*(CV+.1)) [11]
where CV is the weight of all above ground plant material in t ha-1.
| Content Type | Name | Description | Data Type | Unit | Min Value | Max Value | Default Value |
|---|---|---|---|---|---|---|---|
| constant | cCarbonContent | Carbon content of upper soil layer | DOUBLE | % | 0.0 | 20.0 | 0.5 |
| input | iCropResidues | Crop residues plus above ground biomass | DOUBLE | g/m2 | 0.0 | 20000.0 | - |
| input | iLeafAreaIndex | Leaf area index | DOUBLE | m2/m2 | 0.0 | 10.0 | - |
| input | iPotentialEvaporation | Potenial Evaporation | DOUBLE | mm | 0.0 | 12.0 | - |
| input | iPrecipitation | Rain amount | DOUBLE | mm | 0.0 | 60.0 | - |
| input | iRadiation | Solar radiation | DOUBLE | MJ/m2 | 0.0 | 2000.0 | - |
| input | iSoilTempArray | Soil Temp array of last day | DOUBLEARRAY | °C | -15.0 | 35.0 | - |
| input | iTempMax | Daily maximum temperature | DOUBLE | °C | -40.0 | 50.0 | - |
| input | iTempMin | Daily minimum temperature | DOUBLE | °C | -40.0 | 50.0 | - |
| AgeOfSnow | Age of snow | INT | 1 | null | |||
| SnowIsolationIndex | Snow isolation index | DOUBLE | 1 | null | |||
| SnowWaterContent | Snow water content | DOUBLE | mm | null | |||
| SoilSurfaceTemperature | Soil surface temperature | DOUBLE | °C | null |