Guidelines for Precipitation Station

The main purpose of establishing a rainfall monitoring station is to obtain representative samples of the rainfall over a basin. Site selection for the rain gauge instrument is critical for obtaining a representative sample. Conditions around the gauge affect its effectiveness in collecting a sample that accurately reflects the area that it is designed to represent. Particularly, wind affects the rainfall measurements, while further losses due to evaporation and splashing also play a role. To eliminate or reduce wind effects the site should be chosen such that the wind speed at the level of the rain gauge is as low as possible, but in such a way that the surrounding environment does not significantly affect the rain catch.

Ideally, protection against the force of wind should come from all directions by objects of uniform height. Trees, shrub, etc. of nearly uniform height are ideal to protect the gauging site from wind, provided that the angle from the top of the gauge to the top of the encircling objects and the horizontal is between 30o and 45o. Windbreaks of a single row of trees or a building (currently present or to be constructed) should be avoided as they tend to increase the turbulence (WMO, 1994).

Slopes also affect the wind field. Sites on a slope or with the ground sloping sharply away in one direction (particularly in the direction of the wind) are to be avoided.

The gauge should be on level ground above flood level and free from water logging. Further, the site should have the same ground cover as the natural cover in the surrounding area. Surroundings covered with short grass are ideal. A hard ground such as concrete gives rise to excessive splashing and should be avoided. The plot required for an automatic rain gauge (ARG) station is 10 m x 5 m.

Ideally, the rain gauge should not be installed on top of buildings as per WMO guidelines. However, if the land availability and site security are the consideration, the rain gauge may be installed on roof of building in some specific cases.

The site should be away from any obstruction. If there is a tree or a building nearby, the distance of rain gauge should be at-least twice the height of building. (L>2H, where H is height of obstruction; and L is distance of rain gauge from obstruction). See the figure below for explanation.

Establishing and maintaining datum is critical for stage monitoring as it provides the underlying basis for the calibration of the sensors. Thus benchmarks should be located within approximately 100 m of the proposed site to serve as the basis for gauge datum. If no such benchmarks currently exist at the site, plans should be made to establish a new benchmark using survey grade GPS equipment.

Snow Pillow Sites: Per NRCS (2011) guidelines, a 60 m area is recommended to locate the snow pillow and accompanying equipment and to provide an adequate window for solar recharge and telemetry communication. Factors to consider in site evaluation include physical site characteristics (e.g., slope, aspect, exposure, vegetative cover, geographical area representation, access, and footprint), communication characteristics (e.g., telemetry and solar window), security, aesthetics, and access for manual collection of data during snow surveys. In particular, vegetation, physiographic characteristics, and snowfall patterns should be representative of the area. Slope at the site should not exceed 10o and care should be taken that the site is not located in avalanche prone areas. Aspect is important as north facing slopes generally retain snow longer thus provide longer periods of persistence in snow. Exposure to wind should be minimized to limit the effects of drifting snow accumulation thus over measuring the precipitation. Year round access is recommended for maintenance.

Several factors are to be taken into consideration while making a proper choice for the site of the observation station to ensure long-term reliable data:

• Spatial distribution: approximate positioning of the station on the map to obtain maximum reduction of estimation errors, in relation with the variable of concern and observation interval to be used.
• Integration: sites should be selected in context with existing or planned hydrometric and/or groundwater networks. If other hydro-meteorological stations are operated by agencies other than the HIS implementing agency, and iftheir stations fit into the HIS network and their equipment and operational performance meet the HIS standards, then discussions should be initiated with those agencies to get access to their data on a regular basis.
• Technical aspects: variables to be measured, where and with what accuracy and frequency, integration with surface water and/or groundwater quantity and quality networks
• Environmental aspects: availability of suitable levelled ground, exposure conditions, future expansion near the site, no water logging,
• Logistical aspects: accessibility, communication, staffing,
• Security aspects: security of instruments, location in relation toresidential areas and play grounds,
• legal aspects: land acquisition,rite of passage, and
• Financial aspects: including costs of land acquisitioncivil worksequipmentdata processing, staffing and training.

Questionnaire

1. Is the distance of rain gauge at-least twice of the height of nearest obstruction?
2. Are there any strong winds?
3. Is the ground surrounding the rain gauge a flat ground having no slopes?