RN-44 : Rainfall simulator studies

Abstract

Hydrologic analysis of watershed response to rainfall carried, out using lumped models, or mathematical models are quite common. These models involve certain parameters determined from the historical data and are subjected to data error. In order to evaluate the performance of a model and to improve upon the hydrologic modelling techniques suitable for the field it is necessary to acquire accurate data by means of artificial creation of natural condition and measurements as accurately as possible is essential. This not only helps in evaluating and improving hydrologic model but also in understanding the hydrologic response of a watershed and the effect of various measures taken in watershed management . or soil conservation etc.

Several types of simulators have been developed in the past for artificial application of rain.These studies were mainly concerning soil erosion. Rainfall simulators starting from simple spray of water using garden cans to programmable and portable nozzle type simulators have been reported in literature.

The simulators aim at either producing uniform drops which are stationary or producing non-uniform drops. Meyer-McCune Rainulator developed in 1958 is a nozzle type simulator and, has been widely used by U.S. Department of Agriculture. This simulator underwent many improvements but essentially keeps the original features until today. A nozzle type simulator known as Type F was developed by U.S. Soil Conservation Service. Since this was expensive, a type FA simulator was developed subsequently and used.

Use of capillary tubes for drop production in Abe literature. Notable amongst them are Chow's model using polythene tubes id UOR's use of hypodermic needles. At Indian Institute of Technology, New Delhi a battery of capillary tubes were used for soil erosion studies. A technique combining air jet around capillary tube was tried by Onstad in 1981 following Hamon who originally developed such an apparatus in 1979.

Automobile tire was rotated to create turbulent air boundary layer. Water jet interaction with this boundary layer was used by Dreszenso in 1981 for rainfall production. This is expected to aid the laboratory calibration of meteorological instruments used to measure certain aspects of rainfall.

Further studies of rainfall simulators which could produce non-uniform size droplets are needed since rainfall in nature invariably consists of several drops of non-uniform size. The areas of further research shall be towards:

i. Studies on water jet and air turbulent boundary layer around a rotating whell for indoor simulation,

ii. studies on the methods of characterising the non-uniform size drops simulated involving collection and measurement of sizes and velocity of samples;

iii. similar characterisation of natural rain region wise which need to be simulated;

iv. the use of air jet around capillary tube to produce desired rainfall;

v. the use of nozzle and automation of the production of desired characteristics of rainfall such as size distribution fall. velocity intensity, etc.