Detailed Responses to FAQ

General

• How do I submit a FAQ?
  - For now, just send a consise message to "globe@hwr.arizona.edu" (no quotes). Questions will be answered sometime over the weekend or monday before noon (PST).
• How do we submit results from "experimental" protocols?
  - Please send me your test data and any other information about your site and procedures that I might need to know. My fax is: 520- 621-1422.

Site Selection

• Why don't you want the site to be irrigated?
  - The purpose of these soil moisture sites is to assess the soils natural capability to hold moisture and the current soil moisture status. Irrigated soils are rarely representative of large areas and represents a very artificial condition - one that has almost no bearing on the sites solar and precipitation history.
• What are optimal characteristics for a soil moisture site?
  - Climatologically useful sites will be broadly characteristic of a major land class / soil unit. In general, they will be natural (minimal influence from irrigation and activities such as mowing) and likely to remain in this state for many years (long- term records are best made away from rapid development/land- use change). The slope should be minor and depressions (where ponding might occur) are to be avoided. It is extremely valuable to have a rain gauge within 1 km and other local meteorological observations are also useful. The site must be unirrigated.

Procedures

• What other equipment can I use?
  - I am still working on this. Hopefully I will have a design out this spring that will allow you to build a curcuit to go with a common Digital volt meter for $50- $100.
• What happens to the gypsum block when the ground freezes?
  - Basically I would not trust the gypsum blocks when the ground is frozen but I do not really have experience in this area as frozen soil is something I do not have to deal with in Tucson. I would like you to try to dig or core some soil moisture samples once a month this winter and ask the questions:
  - does the soil moisture decrease during the winter?
  - how deeply does the ground freeze?
  - what is the best way to dig/core in frozen ground?
  Keep track of your results - they will be valuable for new protocols I am developing for Phase II!
• How do we make a calibration curve if our soil is always wet?
  - This is quite common. Your soil will probably dry out during the summer but you might not be around to observe it. Ask yourself if the site you have selected is characteristic of your area or did you find the only bog for miles around (in which case you might want to move the site). One thing to do is try taking some of your soil inside and setting up a "laboratory" calibration. See this web site for more information.
• Why do I get negative meter readings?
  - The way gypsum blocs work is that they saturate easily in most soils, eg they give a constant high meter reading. As the soil dries out fairly completely, the gypsum block reading dips lower, until when it is oven dry, the block often reads negative, which is analogous to an open circuit, which can also be due to a broken wire. If your readings "come back" after it rains, you don't have to worry about broken wires.

Common Mistakes

• Using a common multimeter to make the readings
  - A DC multimeter cannot be used to measure the gypsum blocks as an AC resistance is what is needed. The Delmhorst digital meter is recommended but is pricey at $245. I suggest you wait until this spring when I will have an alternative black box to hook in to your DVM that makes the bridge measurement at a much lower cost.
• Mixing up soil moisture wires
  - Be sure that the wires coming from different depths are clearly labeled and, if possible, physically separated. The Guide's suggestion to have individual PVC collars over your wires works best - then just make up a form that is filled in in the same order that your PVC collars are on the ground.
• Dividing by wet weight rather than dry weight
  - You are no doubt confused about this issue - we have flip-flopped a couple of times on this issue just in case you weren't. Soil scientists divide by the dry weight to calculate the soil water content, in part to normalize by the relatively constant dry soil mass as opposed to the ever changing total soil plus water mass.

Activities

• How can I share an activity developed at my school?
  - Send it to me and I will try to get it up on the web. I always enjoy hearing about your progress (and problems).
• A simple model of soil moisture- soil interaction?
  - To investigate why a low 60cm meter reading takes so long to increase after a long dry period, have the students use the 2mm sieve and build a soil column in a clear plastic tube 1 m long.
  • Sieve dry soil into the tube.
  • Predict the vol. of water it will take to wet 10,30 60 and 90 cm.
  • Calculate the volume of water you need to pour in the tube to "fill" it to a depth of 10 mm, ie equivalent to a 10mm precipitation event.
  • Pour in 10 mm equivalent volume of water. What happens?
  • Continue adding 10 mm increments, waiting to see what happens each time.
  • What happens and why? Share your findings with me. Look at your data - does it make more sense?