13 models can simulate the complete runoff regime, providing multiple outputs e.g.
river discharge, phreatic surface level and evaporation loss while black box models can offer only one output. In these models transfer of mass, momentum
and energy are calculated directly from the governing partial differential equations which are solved using numerical methods. As the input data and computational
requirements are enormous, the use of these models for real-time forecasting has not reached the ‘production stage’ so far.
Physically-based distributed models can in principle be applied to almost any kind of hydrological problem. Some examples of typical fields of application
are catchments changes, ungauged catchments and spatial variability.
2.2.3 Hydrometeorology Component
Precipitation Precipitation is the primary input vector of the hydrologic cycle and the
most important input into a simulation model of the land phase of hydrological model. Precipitation is derived from atmospheric water, its form and quantity thus
being influenced by the action of the climatic factor such as wind, temperature and atmospheric pressure.
Interception Interception is defined as the process whereby precipitation is retained on
the leaves, branches, and steams of vegetation and on the litter covering the ground.
14 Some observed percentages of interception by various crops and grasses
shown as Table 1. Table 1. Percentages of Interception by Various Crops and Grass
Vegetation Vegetation Type
Intercepted
Crops Alfalfa
Corn Soybeans
Oats 36
16 15
7 Grasses
Little Bluestem Big Bluestem
Tall panic grass Bindweed
Buffalo grass Blue grass
Mixed species Natural grasses
50-60 57
57 17
31 17
26
14-19
source: O. R. Clarck 1937
Evaporation and Transpiration Evaporation is the transfer of water mass from the liquid to the vapor state.
Transpiration is a plant metabolism process where water is received from the soil and released as vapor to the atmosphere. Evapotranspiration take place from
water, snow, land, and vegetation surface. The volume of water which leaves the land phase of the hydrologic cycle by actual evaporation and transpiration in the
most cases exceeds that which flows to the oceans by runoff.
Radiation The processes by which the sun’s energy reaches the surface of the earth
are complex. Radiation is one form of energy transfer. From a hydrologic viewpoint, the important forms of solar energy reaching the surface of the earth
are: shortwave solar radiation consisting of direct solar radiation, scattered sky
15 radiation, and long-wave atmospheric radiation, all in downward direction; and
reflected components of short-wave radiation and the long-wave terrestrial radiation emitted by the surface of the earth.
Wind speed and direction Both wind speed and direction are important for calculating another
atmosphere processes. Wind speed is important for calculating evaporation, snow melt, or rain gauge loses. Wind direction can be used to estimate flow of
water vapor.
Cloud cover Cloud cover affects the transfer of energy to the land surface by
intercepting part of the direct sort-wave radiation. Cloud cover observations are important in situations where no observations exist on incoming radiation. In
such cases, estimated values of clear sky radiation are used with adjustments for affect of cloud cover.
Stream flow Stream flow records provide a measure of the response of a catchments to
the time variable input and internal hydrologic processes. These records are used in simulation techniques during model calibration to assess the dominant
processes contributing to the response of base flow and overland flow. Figure below describe the relationship of each hydrologic component.
16 Figure 3. Relationship of each Hydrologic Component
Source : http:www.sbg.ac.atgeoidrisigis_environmental_modeling sf_paperscollins_fredcollins.html
2.2.4 Classification of Hydrology Modeling
