Downsizing a long-term precipitation network: Using a quantitative approach to inform difficult decisions

<div><p>The design of a precipitation monitoring network must balance the demand for accurate estimates with the resources needed to build and maintain the network. If there are changes in the objectives of the monitoring or the availability of resources, network designs should be adjusted. At the Hubbard Brook Experimental Forest in New Hampshire, USA, precipitation has been monitored with a network established in 1955 that has grown to 23 gauges distributed across nine small catchments. This high sampling intensity allowed us to simulate reduced sampling schemes and thereby evaluate the effect of decommissioning gauges on the quality of precipitation estimates. We considered all possible scenarios of sampling intensity for the catchments on the south-facing slope (2047 combinations) and the north-facing slope (4095 combinations), from the current scenario with 11 or 12 gauges to only 1 gauge remaining. Gauge scenarios differed by as much as 6.0% from the best estimate (based on all the gauges), depending on the catchment, but 95% of the scenarios gave estimates within 2% of the long-term average annual precipitation. The insensitivity of precipitation estimates and the catchment fluxes that depend on them under many reduced monitoring scenarios allowed us to base our reduction decision on other factors such as technician safety, the time required for monitoring, and co-location with other hydrometeorological measurements (snow, air temperature). At Hubbard Brook, precipitation gauges could be reduced from 23 to 10 with a change of <2% in the long-term precipitation estimates. The decision-making approach illustrated in this case study is applicable to the redesign of monitoring networks when reduction of effort seems warranted.</p></div

Boxplots of monthly precipitation (mm day^-1) for the 1998 to 2014 period.

<p>Boxes extend from the 1^st to the 3^rd quartile and the line shows the median. The whisker extends up to 1.5 times the and interquarile range (IQR) from the box or to the most extreme value. Open circles indicate values more than 1.5 times the IQR from the box. Vertical dashed lines separate the north and south facing gauges. Colored boxes highlight gauges discussed in the text, with colors corresponding to <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0195966#pone.0195966.g006" target="_blank">Fig 6</a>.</p

Location, elevation, and date of establishment of precipitation gauges at the Hubbard Brook Experimental Forest, New Hampshire, USA.

<p>Volume is the average annual precipitation from 1998 to 2014. Note that RG18 was decommissioned in 1975. RG22 is not included in this analysis because it is located far from the gauged catchments (at the U.S. Forest Service headquarters). Thus there are 23 precipitation gauges in this study.</p

Downsizing a long-term precipitation network: Using a quantitative approach to inform difficult decisions - Fig 5

<p>The impact of each reduction scenario (Figs <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0195966#pone.0195966.g003" target="_blank">3</a> and <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0195966#pone.0195966.g004" target="_blank">4</a>) was summarized across the six south-facing catchments (A) and three north-facing catchments (B) by calculating the mean absolute deviation across the catchments. Red numbers in parentheses indicate the number of unique scenarios for each number of remaining gauges.</p

Estimates of annual precipitation (P), evapotranspiration (ET), sulfate bulk deposition, and calcium bulk deposition for W6 based on the reduced precipitation monitoring scheme compared to estimates based on the full monitoring network.

<p>Estimates of annual precipitation (P), evapotranspiration (ET), sulfate bulk deposition, and calcium bulk deposition for W6 based on the reduced precipitation monitoring scheme compared to estimates based on the full monitoring network.</p

The effect of removing gauges for each of the three north-facing catchments is shown as the absolute value of the deviation of annual precipitation estimates for all possible combination of gauges.

<p>The results are shown as a function of the number of gauges remaining. The deviation from annual precipitation is based on the IDW method. The green line indicates zero deviation.</p

Cumulative frequency distributions of the catchment-scale precipitation deviations between estimates based on the reduced 6- gauge network (1, 2, 4, 6, 9, 11) for the south-facing catchments and estimates based on the full (12-gauge) network.

<p>The black line shows daily deviations and the blue line shows annual deviations. Only days with at least one gauge reporting precipitation were included in the daily deviation calculation. We used the precipitation record from 1964 to 2014 (the common time period when all gauges were in operation). The scale for annual deviations is at the top and for daily deviations is at the bottom. Note that the direction of skewed daily deviations, most visible in the heavy tail, corresponds with the direction of bias in the annual deviations.</p

Downsizing a long-term precipitation network: Using a quantitative approach to inform difficult decisions - Fig 6

<p>Spatial interpolation of precipitation for south-facing (a,b) and north-facing (c,d) catchments using the inverse distance weighting (a,c) and Thiessen polygon (b,d) methods. Open circles indicate gauges that were eliminated and closed circles indicate gauges that were retained.</p

Map of precipitation gauges (designated RG1 through RG25, excluding RG18 and RG 22).

<p>Rain gauges were located to best characterize precipitation inputs to small headwater catchments (designated W1 through W9) monitored for streamflow at the at the Hubbard Brook Experimental Forest.</p

The effect of removing gauges for each of the six south-facing catchments is shown as the absolute value of the deviation of annual precipitation estimates for all possible combination of gauges.

<p>The results are shown as a function of the number of gauges remaining. The deviation from annual precipitation is based on the IDW method. The green line indicates zero deviation.</p