Automatic Weather Stations for Network Operation

Measurements of evaporation were made from July to November 1999 using the eddy correlation method on two wetland types -wet grassland and reedbeds -in south west England. The evaporative water use of a reed bed exceeded that of the grassland wetland by 15% (or 50mm over the 5 months). The evaporation rates at both sites exceed of the Penman Potential Evaporation estimates calculated for this area. The difference between sites results from the higher roughness length of the reed bed and the lower effective surface resistance of the reed/open water assemblage. At the grassland site, a significant relationship between the surface resistance and water table level has been demonstrated. The water table at this site is managed to maintain the plant diversity and allow some agricultural access. This regime specifies a water table below the surface during the summer period, which results in higher surface resistances and lower evaporation. The results have important implications for local water resources management, especially where wetlands are maintained by pumping from rivers or groundwater.

Section: Equipmentmentioning

confidence: 99%

Evaporation characteristics of wetlands: experience from a wetgrassland and a reedbed using eddy correlation measurements

Acreman

Harding

Lloyd

et al. 2003

“…IH was a leader in automatic weather station development (McCulloch and Strangeways, 1966;Strangeways, 1972) and new opportunities for field studies were the direct result. Creating the capability to make such measurements without the need for a human observer other than to provide occasional service visits meant that it became possible to monitor climate in remote locations where regular manual data capture was impossible (e.g.…”

Section: Routine Measurement Of Weathermentioning

confidence: 99%

Putting the "vap" into evaporation

Shuttleworth¹

2007

215

147

In the spirit of the Special Issue of HESS to which it contributes, this paper documents the origin and development of the science of natural evaporation from land surfaces over the last 3035 years, since the symposium A View from the Watershed was held to commemorate the opening of the new Institute of Hydrology (IH) building in 1973. Important subsequent technical progress includes the ability to measure routinely the diurnal cycle of near-surface meteorological variables using automatic weather stations, and of surface energy and momentum exchanges using automated implementations of the Bowen Ratio/Energy Budget technique and the Eddy Correlation technique, along with the capability to estimate the fetch for which these measurements apply. These improvements have been complemented by new methods to measure the separate components of evaporation, including: the interception process using randomly relocated below-canopy gauges, transpiration fluxes from individual leaves/shoots using porometers and from plants/plant components using stem-flow gauges and soil evaporation using micro-lysimeters and soil moisture depletion methods. In recent years progress has been made in making theory-based areaaverage estimates of evaporation using scintillometers, and model-based area-average estimates by assembling many streams of relevant data into Land Data Assimilation Systems. Theoretical progress has been made in extending near-surface turbulence theory to accommodate the effect of the excess boundary layer resistance to leaf-to-air transfer of energy and mass fluxes relative to that for momentum, and to allow for observed shortcoming in stability factors in the transition layer immediately above vegetation. Controversy regarding the relative merits of multi-layer model and big leaf representations of whole-canopy exchanges has been resolved in favour of the latter approach. Important gaps in the theory of canopy-atmosphere interactions have been filled, including recognising the need, separately, to represent dry-canopy and wet-canopy evaporation in models and the capability to describe wet-to-dry canopy transitions as well as the ability to describe sparse vegetation canopies which only partly cover the underlying soil. There is progress in methods of estimating crop water requirements, but an important recommendation of this paper is that this progress should continue by introducing use of an effective stomatal resistance rather than crop factors. The paper draws attention to relevant theoretical insight on this issue. Progress in theoretical understanding of evaporation processes has been used in the creation of numerous Land Surface Parameterisations (LSPs), the models used to represent land-surface interaction in climate and weather forecast models, and there have been important advances in describing the behaviour of plant stomata in LSPs. A major investment over the last 25 years in conducting Large-Scale Field Experiments, the better to measure, understand and model coupled land-surface/atmosphere intera...

“…The need to develop instrumentation capable of reliable operation in remote, inhospitable field conditions was recognised as a priority and a programme of instrument development was initiated. The production of design concepts and prototypes which were then developed by private sector companies led to much of the array of field instrumentation taken for granted today, such as automatic weather stations (Strangeways and McCulloch, 1965;Strangeways, 1972) and neutron soil moisture meters (Bell and McCulloch, 1969), and to the adoption of solar powered solid state loggers as standard data collection platforms. As they were proven, these developments were incorporated in the Plynlimon networks.…”

Section: Instrument Development For a Rigorous Scientific Approachmentioning

confidence: 99%

“…Considerations of security and the area required for acceptable exposure restricted the East African studies to one meteorological site, read twice daily, per catchment. In the UK by contrast, the work on instrument development meant that it was possible to deploy networks of Automatic Weather Stations (Strangeways, 1972) over the altitude range in each of the Plynlimon catchments at an early stage in the study. As well as the vast improvement in sampling frequency, these had the advantage of providing direct measurements of net radiation.…”

Section: Potential Evaporationmentioning

confidence: 99%

Development of catchment research, with particular attention to Plynlimon and its forerunner, the East African catchments

Blackie¹,

Robinson²

2007

Dr J.S.G. McCulloch was deeply involved in the establishment of research catchments in East Africa and subsequently in the UK to investigate the hydrological consequences of changes in land use. Comparison of these studies provides an insight into how influential his inputs and direction have been in the progressive development of the philosophy, the instrumentation and the analytical techniques now employed in catchment research. There were great contrasts in the environments: tropical highland (high radiation, intense rainfall) vs. temperate maritime (low radiation and frontal storms), contrasting soils and vegetation types, as well as the differing social and economic pressures in developing and developed nations. Nevertheless, the underlying scientific philosophy was common to both, although techniques had to be modified according to local conditions. As specialised instrumentation and analytical techniques were developed for the UK catchments many were also integrated into the East African studies. Many lessons were learned in the course of these studies and from the experiences of other studies around the world. Overall, a rigorous scientific approach was developed with widespread applicability. Beyond the basics of catchment selection and the quantification of the main components of the catchment water balance, this involved initiating parallel process studies to provide information on specific aspects of catchment behaviour. This information could then form the basis for models capable of extrapolation from the observed time series to other periods/hydrological events and, ultimately, the capability of predicting the consequences of changes in catchment land management to other areas in a range of climates.