Concept:

Precipitation- It is the fall of water in various forms on the earth from the clouds.

The usual forms of precipitation are as follows:

Note:

Horton's Formula:

Infiltration is the process in which water enters into the soil and the rate at which water enters into the soil is known as infiltration rate. Under the ponding condition, the infiltration rate decreases exponentially, and it can be represented by the Horton equation. According to Horton’s equation,

$\mathrm{f}={\mathrm{f}}_{\mathrm{c}}+\left({\mathrm{f}}_0-{\mathrm{f}}_{\mathrm{c}}\right)\times {\mathrm{e}}^{-\mathrm{k}\mathrm{t}}$

Where,

f = infiltration capacity at time t, f0 = initial infiltration capacity, fc = constant infiltration capacity after long time, and k= Horton’s decay constant

Lysimeter:

A Lysimeter is a measuring device that can be used to measure the amount of actual Soil moisture lost from the soil column (indirectly actual evapotranspiration) that is released by plants (usually crops or trees). By recording the amount of precipitation that an area receives and the amount lost through the soil, the amount of water lost to evapotranspiration can be calculated. Lysimeters are of two types: weighing and non-weighing.

Blaney-Criddle method: 

It is used to determine evapotranspiration i.e. consumptive use of the crop.

It is one of the oldest methods to measure evapotranspiration (Consumptive use of water).

Pan coefficient:

It is the ratio of lake evaporation and pan evaporation.

In order to convert pan evaporation value into lake evaporation value, a pan coefficient is used.

Lake evaporation = Pan Coefficient (Cp) × Pan Evaporation

Concept:

Methods use to convert point rainfall values at different rain gauge stations into an average value over a catchment are:

1. Arithmetical mean method
2. Thiessen Polygon method
3. Isohyetal Method

The arithmetical mean method is very crude method and it is rarely used.

Thiessen Polygon method is superior to the arithmetic average method as some weightage is given to various stations on a rational basis.

The Isohyetal Method is considered as the best method as compared to the above two.

Cloud seeding is a type of weather modification that aims to change the amount or type of precipitation that falls from clouds by dispersing substances into the air that serve as cloud condensation or ice nuclei, which alter the microphysical processes within the cloud. The usual intent is to increase precipitation (rain or snow),

The use of cloud seeding is very controversial. Some people say it isn't worth the resources, because even the most dramatic rainfall increases only amount to 5-20%. If it is done incorrectly, it can even reduce the amount of rain that might have fallen by dissipating clouds

Water that percolates through the soil emerges as the dry weather flows in streams.

Statement 1 & 3 are correct.

Explanation:

A rain gauge is an instrument used to measure rainfall by collecting all the precipitation which comes down on an area.

The rain gauges can be broadly classified as:​

IS: 4987 - 1968 recommends the following density of rain gauges:

Concept:

ϕ – index:

t is defined as the ratio of total infiltration when rainfall excess occurs to the total rainfall excess time.

It represents the amount of rainfall that infiltrated into the ground or lost due to trapping etc.

In a hyetograph i.e. intensity v/s duration curve,

 → The area above φ –index is total runoff.

Total runoff = (excess rainfall intensity- φ index) × duration

 ϕ = (Rainfall - Runoff )/ Duration

Calculation:

Given,

Area of catchment = 2 hectares = 2 × 10⁸ cm²

Volume of water = 1180 m³ = 1180 × 10⁶ cm³

Depth of water at outlet = Vol./Area

 = 1180 × 10⁶/ (2 × 10⁸) = 5.9 cm

Total precipitation (P) is :

 P = 0.5 × (1.0 + 2.5 + 4.4 + 3.8 + 2.0 + 0.5)

 = 7.1 cm

w - index = (7.1 - 5.9)/3 = 0.4 cm/hr

As no reading is below 0.4 cm/hr

∴ Φ = 0.4 cm/hr

Trick:

Instead of using Different trials, it is good here to use the reverse approach and checking ϕ – index from options.

Concept:

Hydrograph is used to measure runoff discharge.The area under the discharge and time curve gives the volume of runoff.

Calculation:

Catchment area = 777.6 km² = 777.6 × 10⁶ m²

Total volume of water = Area of Hydrograph

Total volume $=\frac{1}{2}\times 72\times 60\times 3600$

= 7776000 m³

Runoff storm $=\frac{Volume}{Area}=\frac{7776000m^2}{777.6\times {10}^6{m}^2}$

Runoff storm = 10^-2 m

∴ Runoff storm = 1 cm

Explanation:

DAD curve

The maximum depth area curve for a given duration D is prepared by assuming the area distribution of rainfall for smaller duration to be similar to the total storm.

The procedure is then repeated for different storms and the envelope curve of maximum depth area for duration D is obtained.

A similar procedure for various values of D results in a family of envelope curves of maximum depth vs area, with duration as the third parameter. These curves are called DAD curves.

Note:

The shape of the DAD curve is also concave which shifts outward with an increase in duration.

Explanation:

Hydrograph:

(i) It is the graphical representation of runoff with time at the given point on the stream.

Runoff:

(i) It is defined as the flowing away of precipitation from a catchment.

(ii) The flow which takes place all the time over the surface as overland flow and through the channels as open channel flow is surface runoff.

Runoff = Direct runoff + Base flow

Direct runoff : This is the part of runoff which immediately reaches the stream after precipitation. This includes surface runoff, prompt interflow and precipitation on the stream surface.

Base flow = Total runoff - Direct runoff

Direct Runoff hydrograph:

(i) The hydrograph obtained after deducting the base flow from the flood hydrograph is known as DRH.

Unit Hydrograph:

(i) It is a DRH resulting from one unit depth of rainfall excess (Direct runoff) occurring uniformly over the basin at a uniform rate for a specified duration of ‘D’ hours.

The theory of unit hydrograph was developed by Sherman in the year 1932 and assumptions are given below-

1. Time invariance-

• According to this assumption the DRH for a given effective rainfall is always the same in the catchment irrespective of the time, when the rainfall or storm takes place.

2. Linear response-

• This is the single most important assumption in the theory of unit hydrograph. According to this assumption any change in the input value is proportionately reflected in the output value.

Concept:

By applying a thin chemical film on the water surface, the evaporation can be reduced. Currently the application of the chemical over the water surface is the only feasible method available for reduction of evaporation of reservoirs to moderate extent.

Certain chemicals such as cetyl alcohol (hexadecanol) and stearyl alcohol (octadecanol) form monomolecular layers on a water surface and these layers act as evaporation inhibitors by preventing the water molecules to escape past them.

The thin film formed has the following desirable features:

1. The film is strong and flexible and does not break easily due to wave action.

2. If punctured due to the impact of raindrops or by birds, insects, etc., the film closes back soon after.

3. It is pervious to oxygen and carbon dioxide: the water quality is therefore not affected by its presence.

4. It is colourless, odourless and nontoxic.

Dicken’s Method

It was adopted for Northern India.

$Q_P=C_D{A}^{\frac{3}{4}}$

where

Q_P = Peak discharge in m³/s.

A = Area in km².

C_D = coefficient applicable in the region.

Important Points:

Different empirical method for determining flood discharge are as follows:

Ryve’s Method

$Q_P=C_R{A}^{\frac{2}{3}}$

This formula is used only in Southern India.

Jarvi’s Method

This formula is applicable for Eastern India.

$Q_P=C\sqrt{A}$

Inglis Method

This formula is used only in Maharashtra. Here three different cases are taken into consideration.

$Q_P=\frac{124A}{\sqrt{A+10.4}}\cong 123\sqrt{A}$