Q2 Spatial Distribution

Question 1

Explain how different factors affect the spatial distribution of precipitation at different scales and what impact this has on engineering?

Answer 1

Introduction

• formed when a body of warm moist air is cooled

– saturated with water vapour and condenses
essential component of water budget
Knowledge of spatialtemperal distribution

– better understanding and modelling of floods, droughts, landslides and snow avalanches as well as regional climate change

• Point to point gauge measurements
• Notoriously hard to measure accurately and to compound this extremely variable over catchment.

How do they affect it at different scales?

• amount of precipitation varies spatially and with time
• Static (don’t vary between storm events) and dynamic influences (variations in weather)
• Global scale mainly dynamic
• Continental scale – mixture of both e.g USA northwest (cyclonic), Florida (warm Caribbean sea)
• Small scales static factors dominant
• Although convective thunderstorm (Essex 2000) 10mm rainfall in one hour

What factors affect the distribution of precipitation?

1) Static Influences

• easier to account for

Altitude

• temperature crucial factor control water vapour capacity
• Orographic precipitation ( strong correlation between altitude and rainfall)

Aspect

• less important than altitude
• Slopes within a catchment that face eastwards will naturally be more shelters from the rain than those facing westwards. The same principle applies everywhere: slopes with aspects facing away from the predominant weather patterns will receive less rainfall than their opposites.

Slope

• only relevant at very small scale
• measurement occurs at small scale (rain gauge)
• The differences difference between a level rain gauge on a hill slope, compared to one parallel to the slope, may be significant. It is possible to calculate this difference if it is assumed that rain falls vertically. Consequently the effect of the slope on rainfall measurements is normally ignored.

2) Rain Shadow effect

• Where there is a large and high land mass it is common to find the rainfall considerably higher on one side than the other. This is through a combination of altitude, slope, aspect and dynamic weather direction and can occur at many different scales.

South Island of New Zealand

The predominant weather pattern for the South Island of New Zealand is a series of rain-bearing depressions sweeping up from the Southern Ocean, interrupted by drier blocking anticyclones. The southern Alps form a major barrier to fast-moving depression hand have a huge influence on the rainfall distribution within the South Island. The predominant weather pattern has a westerly airflow bringing moist air from the Tasman Sea onto the South island. As this air is forced up over the Southern Alps it cools down and releases much of its moisture as rain and snow.. The rain shadow effect can be clearly seen with west coast rainfall being at least four times that of the east.

3) Forest Rainfall Partitioning

• Once rainfall falls onto a vegetation canopy it effectively partitions the water into separate modes of movement: throughfall, stemflow and interception loss

Throughfall

• water falls to the ground directly through gaps in the canopy or indirectly dripping of vegetation
• direct governed by canopy coverage
• indirect governed rainfall characteristics and canopy storage

Stemflow

• Stemflow is the rainfall that is intercepted by stems and branches and flows down the tree trunk into the soil
• small part of the hydrological cycles (2-10% of canopy rainfall)
• smoother the bark the larger the flow

Interception

• While water sits on the canopy, prior to indirect throughfall or stemflow, it is available for evaporation, referred to as interception loss. It can also be gained for instance during fog.

4) Types of precipitation

• Snow flakes the majority of the time are more easily transported by wind than raindrops.
• When the snow reaches the ground it can be easily blown around in secondary manner(drifting). - - Fog was previously mentioned

Impact on Water Quality

• major input – water quantity and quality
• Rainfall intensities – storm design – irrigation design
• Exception Colorado river flows through Utah and Arizona – quantity governed by precipitation in rocky mountains
• Quantity dilution of contaminents
• Airbourne pollutants dissolved in precipitation ( acid rain (-) transfer nitrous oxide (+)
• Eng design: Avalanche, landslide, floods and droughts
• Measuring spatial distribution to predict future levels