SNOW PACK AND SNOW MELT

2.14 SNOW PACK AND SNOW MELT

Snow melts due to heat from the atmosphere, from the warm rainfall and by radiation. On the basis of measurement of snow depth and water equivalent (or density) of snow at grid stations

HYDROLOGY

along the snow courses, the water storage in the snow (i.e., the available snow melt) can be determined from the formula.

...(2.11) where d w = depth of water storage in the snow

d w =G s d s

d s = depth of snow cover

G s = water equivalent (density) of snow Water equivalent or snow density is the ratio of the volume of melt water from a sample

of snow to the initial volume of the snow sample. An average density of 0.1 for freshly fallen snow is often assumed; however the longer the snow remains on the ground, the denser it becomes due to packing, alternate thawing and freezing, condensation and the presence of absorbed rainfall and melt water. Snow density will be about 0.1 at the beginning of winter and about 0.3 at the end of winter, i.e., at the beginning of thaw. The density of snow varies with depth and so samples must be taken at various horizons in a snow pack for determining the water equivalent. This is usually done by a sampling tube. Snowfall may be measured directly by an ordinary rain gauge fitted with a heating system, or by a simple snow stake, if there is no drifting and density is determined simultaneously.

Snow surveys are made by dividing the basin into rectangular grids and determining the snow depths and densities at depths at the grid stations. From the water equivalents thus obtained at individual stations, the average water equivalent of the snow pack over the entire basin can be derived by the same method used for determining the mean areal depth of rainfall. In India, snow surveys are generally conducted in the beginning of summer, i.e., in March or April, when snow starts melting. Many of the perennial rivers in north India are fed by snow- melt from the Himalayan range in summer. In the Himalayan range, one-tenth of the depth of snow is added to the depth of rainfall, as a thumb rule, to obtain the total precipitation.

Also see Appendix-A.

QUIZ II

I Match the items in ‘A’ with items in ‘B’:

(i) Cyclone (a) Optimum rain-gauge network (ii) Frontal surface

(b) Intensity of storm

(iii) Rain-gauge

(c) Can not record snow

(iv) Double mass curve

(d) 1 in 500 km 2

(v) Tipping bucket gauge (e) Mean areal depth of precipitation (vi) Recording rain-gauge

(f) Western Ghats (of Peninsular India) (vii) Orographic precipitation

(g) Covergence of storm

(viii) Thiessen network (h) Surface of contact between warm and

cold air masses

(ix) Raingauge density

(i) Air port

(x) Coefficient of variation, C v (j) Change of regime of rain-gauge station

PRECIPITATION

II Fill up the blanks in the following: (i) Rain-gauges are erected...... over the ground surface with their rim at .... cm above ground

surface (ii) The three principal agencies, which maintain rain-gauges and make observations in Karnataka

are: (a) ....... (b) ........ and (c) ...... (iii) Rain-gauge readings are taken every day at .... hr IST (iv) In coastal areas of Karnataka one rain-gauge should be erected for every ..... km 2 and ....... years (consecutive) of record are required for statistical analysis. (v) An index of wetness of 40% in a certain area indicates a ..... year with a rainfall deflciency of

....%. III Say ‘True’ or ‘False’; if false, give the correct statement: (i) Rain-gauges are erected perpendicular to the ground surface on which they are installed. (ii) Cyclonic precipitation is due to convergence of storms towards a low pressure belt. (iii) In hilly and heavy rainfall areas at least 10% of the rain-gauges should be of self-recording

type. (iv) As the area increases, the average depth of precipitation increases for a particular storm. (v) Generally, high intensity precipitation can be expected only for short durations, and higher

the intensity, lesser is its frequency. (vi) The coefficient of variation for annual precipitation data is equal to the standard deviation of

the indices of wetness. (vii) A mass curve of rainfall need not always be a rising curve. (viii) The longer the snow remains on the ground, the less dense it becomes. (ix) The intensity of storm is an inverse function of its duration.

(false: i, iv, vii, viii) IV Choose the correct statement/s in the following: 1 Precipitation includes (i) rainfall

(ii) snow melt

(iii) hail storm

(iv) stream flow

(v) mist and fog

(vi) frost

(vii) all the above 2 In a cold front (i) cold air mass drives out a warm air mass (ii) warm air mass replaces the retreating cold air mass (iii) cold air and warm air masses are drawn simultaneously towards a low pressure area

(iv) the cold and warm air masses are stationary 3 Cyclonic precipitation is due to (i) orographic lifting

(ii) ocean nearby

(iii) convergence of storms towards a low pressure belt (iv) divergence of storms

(v) thermal convection

(vi) conflict between cold and warm air masses 4 A self-recording rain-gauge (i) records by hourly depth of rain

(ii) records the snow melt

(iii) records the cumulative depth of rainfall (iv) records the rainfall intensity

(v) records the onset and cessation of rainfall (vi) records the cloud cover

HYDROLOGY 5 A double mass analysis is made

(i) to find the missing rainfall at a station in a particular area (ii) to detect any change in exposure of a station (iii) to adjust the record at a station to the changed environment (iv) to compute the a.a.r. consistent with the changed environment (v) for all the above purposes 6 Isohyetal method gives accurate mean areal depth of rainfall (i) in a plain country (ii) in a gently sloping basin (iii) in an undulating country (iv) in places of known storm movement (v) in a basin consisting of plains and hills (vi) when there are optimum number of rain-gauge stations (vii) when the precipitation includes snowmelt (1-exept iv; 2–i ; 3–iii ; 4–iii, v ; 5–ii, iii, iv ; 6–iii, iv, v)