General

The first phase of the research project involved manual measurements of rainfall, runoff volumes, and soil moisture. This phase continued while automated measurement systems were being developed and tested. The project moved to the second phase when most of the manual sampling procedures were supplanted with automated systems.

The purpose of the second phase was to increase the frequency of rainfall, air temperature, wind speed, relative humidity, solar radiation, runoff volumes, and soil water content measurements so that detailed site-specific data would be available to observe runoff responses to a range of rainfall intensities and soil moisture contents. It was planned to use this more detailed data to refine a daily soil water balance model described in Chapter 4 and developed using the manual data gathered in phase one.

The necessary automation for the collection of high resolution data was provided by the installation of solar-powered weather stations, one each at C1 North, C1 South, C1 East, and C2 East aspects. Each of these stations recorded the previously mentioned meteorological variables as well as soil moisture and runoff volumes. All dates and times related to data gathered at the research area are in New Zealand Standard Time (NZST).

Each station was protected from stock by an electric fence powered by a 12V car battery which was, in turn, recharged by the solar radiation panel supplying power to the weather station. Sensor data was accumulated within the logger memory and downloaded either by directly connecting a laptop computer to the logger or by using a modem which connected to the logger via a cellular network. Due to the remote location of the research area, connection via the latter method often proved to be unreliable and, as a result, was rarely employed. Table 3.4 summarises the sensor layout for each weather station.

Table 3.4 Summary of the logger and sensor inventory for each of the weather stations. Station Name Installation Date Logger Type1 (replacement date) Sensors C1 North 30-10-2007 Campbell Scientific CR800 (17-12-2009) - Hydrological Services TB5 0.2 mm tipping bucket rainfall gauge - Apogee PYR-S Pyranometer - Vaisala HMP50Y

temperature/humidity sensor - Maximum Hall Effect Anemometer - Campbell Scientific CS616 300 mm

TDR probes

- Custom-built 145 mL runoff tipping buckets C1 South 20-11-2007 Campbell Scientific CR800 (06-01-2010) C1 East 04-12-2007 Campbell Scientific CR211 (14-01-2010) C2 East 05-12-2007 Campbell Scientific CR211 (11-03-2010)

1 _{– All loggers were later replaced with Campbell Scientific CR1000 units}

There were two major problems involving data capture using the loggers associated with the weather stations. The first problem quickly manifested itself in the form of a large number of spurious data signals arriving at the loggers from the runoff tipping buckets for each plot. After testing it was discovered that the pulsed output from the electric fence surrounding each weather station was interfering with the data cables between the logger and the runoff tipping buckets. Considerable time was expended in attempting to resolve this problem: installation of shielded data cables and re-arrangement of the electric fences resulted in minor improvements. A solution to this first issue was finally achieved with the design and construction of optical pulse filters at Massey University. These were inserted between the tipping bucket data cable and the logger and were installed on 12-11-2009 at the C1North and C2East aspects and on 19-11-2009 at the C1South and C1East aspects.

The second problem manifested itself some time after the installation of the TDR probes and proved to be more difficult to identify and resolve. The following symptoms were observed:

1) The C1 North and C1 South loggers experienced random lockups (a failure to respond to communications and a failure to log data).

2) The internal software monitoring system of the C1 North and C1 South loggers began to record errors.

3) The C1 South logger in particular was severely affected with complete loss of accumulated data occurring at times.

4) The symptoms were more prevalent when the soil was very moist and/or during significant rainfall events.

5) The C1 East and C2 East loggers were completely unaffected.

A number of attempts were made to try and isolate the cause of the problem. These involved:

1) Stepwise removal of the various sensors attached to the logger.

2) Switching off the temporary electric fence surrounding the relevant logger.

3) Consultation with the suppliers of the loggers (Scott Technical Instruments Limited). Trouble-shooting was a time-consuming process due to the random appearance of the symptoms and the isolation of the research site. Following recommendations from Scott Technical, all temporary electric fences surrounding the logger stations were replaced with conventional fences. This resulted in a significantly decreased incidence of lockups and internal logging errors, and there were no further instances of data loss. Those errors that did continue to occur were attributed to permanent electric fences 25 m from the C1 North aspect. It is suspected that the more substantial network of underground cabling associated with the C1 North and C1 South loggers (installation of the TDR probes and a greater number of runoff tipping buckets) served to act as antennas and attenuators for sub-surface current flows generated from the combination of temporary and permanent electric fence

installations. These current flows would be more active during wetter conditions and induce electrical spiking (probably via the TDR power and data cables) in the logger and hence the observed lockups and loss of accumulated data.