Specifications

8
Chapter 2
Subject Review
This chapter discusses in detail the b ackground issues presented in section 1.2. It will do
this by presenting documented examples of current methods used in environmental sensing
projects. Common models used to estimate environmental p roperties will then be presented
followed by a discussion illustrating the need for more collaborative analysis efforts. Finally,
a survey of commonly used products used to measure and store data for these projects will
be pr esented and their features discussed. These discussions will illuminate the apparent
need for a new product.
2.1 Current Methods of Subsurface Property Estimation
The two methods commonly used to estimate soil properties using ET are ground-
based weather stations and remote sensing. Ground stations, termed the conventional
approach in fig. 2.1, measures the ET potential and calculate a new estimated ET value
based on a previous soil moisture value. Remote sensing uses satellite imaging to observe
surface moisture flux and a previous value for soil moisture to arrive at th e estimated ET.
This resulting ET value is then inserted into a model and a new soil moisture value is
obtained [1,5]. Possible models available to extrapolate soil moisture from ET are classified
as budget [6], semi-dynamic [7], and dynamic [8]. Any ET-based estimates always have
a processing lag inherently associated with its final output. This delay is caused by the
need to acquire the previous ET value. Processing also is n ot done on site because the
algorithms are to computationally complex to run on these power limited devices. This
offsets the moisture solution by an entire interval. One ET interval is about one day for the
conventional method and three to fourteen days for the remote sensing approach [9]. Using
the best ET methods to date, the best accuracy attained after extensive d ata processing