Static and Dynamic Data LayersBy David R. MaidmentCenter for Research in Water ResourcesUniversity of Texas at Austin7 February, 2008In working with water resources data and problems, we have two basic things we have to do to describe water properties, such as flow, velocity, water level and water quality, and to describe the water environment, such as streams, rivers, landscapes, aquifers, and cities. The interaction of water properties and the water environment is very complex and can be described only for simplified cases. We do this by defining hydrologic systems using the concept of a control volume to isolate a particular region of space and defining its inputs, outputs and relevant interior water properties. To capture all the information needed to describe a hydrologic system, we need several basic sources of information: point observations of water information, like stream gages, water quality sampling sites, groundwater wells, and weather and climate stations; remote sensing observations, such as Nexrad and satellite and aircraft remote sensing; GIS data such as DEMs, watershed boundaries, stream networks, soil and land cover properties, groundwater wells and aquifers; and hydrologic and weather model information, such as outputs from NARR for time-varying weather and climate, or outputs of Hec-RAS for floodplain mapping. I am in the midst of completing the design for Arc Hydro II which is a customized version of ArcGIS for water resources for which I proposed and published an initial design in 2002.I suggest for Arc Hydro II, that we should adopt the following principles:(1) Make the structure of Arc Hydro II conformal with water web services. Initially this means make the Time Series part of Arc Hydro II conformal with WaterML so that AH II can act:(a) as a target for data acquired through web services (in thecase of AH II implemented on ArcGIS Desktop and functioning as a local data harvester - we have an application built for this already); (b) as a source for time series data related to geographic features (in the sense of AH II implemented on ArcGIS Server). I dont think its necessary that AH II be a full observations data repository because we already have CUAHSI ODM for that (and there are other systems, like Kisters, NWIS, etc that also act as sources of observations data).(2) Adopt a concept of static and dynamic data layers as our thought model for definition of an Arc Hydro II geodatabase. By a static data layer, I mean something that has no time variation and is represented by current ArcGIS data structures. By a dynamic data layer, I mean a data layer that has a definition of both its spatial and its temporal coordinate system, and whose information is indexed against time as well as space. Dynamic data layers can take several forms:(a) As a netCDF layer where the information is stored as an multidimensional array with some dimensions of that array indexing the coordinates in space and time and the other dimensions containing the data values at those coordinate points.The spatial coordinate can be a HydroID or an ObjectID or other reference field that is linked to an appropriate feature class in ArcGIS where its spatial properties are defined (ie to allow for model results stored on unstructured grids like finite element models where the nodes and triangles are spatially indexed but not regularly laid out on a grid).This deals with information defined as fields in the meaning of that term in physics where time indexed information is defined on a spatial continuum.(b) As a Space-Time Table that has a DateTime field that specifies the time point for each record in the table (this can be just a pure table linked to geographic features by a relationship (what I have called an Attribute Series - where geometry is fixed through time and only attributes vary); it can be a feature class (what I have called a Feature Series, where geometry can vary through time as well as attributes); and it can be a Raster Series, where the Raster Catalog has a DateTime field. This deals with time-indexed information defined on discrete spatial feature classes. The present ArcGIS Multidimensional tools are designed to allow (a) and (b) to be interconnected.(c) As an Observations Data Layerwhere the GIS stores the spatial locations of the features and links to the observations that are stored in a related database (e.g. ODM) or are accessed through web services (for which WSDL address is stored as part of the Arc Hydro II geodatabase design. The design of the interface for the Data Access System for Hydrology (DASH) http:/river.sdsc.edu/DASH/ has been very instructive in this regard - it has two i buttons, one for the static GIS data layers as normal, and one for the observations data layers, and it has two .mxd map documents that contain the spatial descriptions of these two different kinds of