Assessing the hydrologic functions of prehistoric grid gardens in north central New Mexico

"Cobble-bordered gravel-mulched plots" ("gridded plots"), the most commonly known type of indigenous agricultural feature in the northern Rio Grande, consist of rectangular fields found on many mesas and river terraces. The research described in this dissertation establishes the fact that gridded plots constitute an irrigation technology, and demonstrates the means by which Pueblo farmers manipulated the landscape in order to increase the agricultural water supply.; In this dissertation the principles by which gridded plots operate are analyzed using conceptual and mathematic models borrowed from physical hydrology. Analysis is conducted in the framework of the "soil moisture mass balance equation" (SMBE) which accounts for all additions to soil moisture, precipitation and "runon", and all losses, runoff, evapotranspiration, and drainage. Each term of the SMBE is analyzed using a conceptual physical model which describes the movement of water. This makes it possible to accurately describe the hydrologic processes occurring on a landscape prior to and after construction of a gridded plot. Once the principles of operation are determined it is possible to use physically-based estimation techniques to determine the changes in quantities of water which would move into or out of a location before and after construction of a plot.; This research was conducted on a group of gridded plots located on an alluvial terrace immediately north of Chimayo, New Mexico, referred to as La Mesita. Two groups of plots, each containing three plots, were intensively mapped, and soil hydraulic data were collected. These data were used to construct digital models of the terrain and the soil, both without gridded plots and with plots. The movement of surface water (runon and runoff) on the landscape was then analyzed using equations derived from a new hydrologic concept called TOPMODEL. Additional techniques were used to estimate rates of water addition and loss from the soil through evapotranspiration and drainage, as well as other runon and runoff processes.; Results of analyses indicate that construction of a gridded plot will promote accumulation and prevent loss of significant quantities of water. Installation of a mulch will prevent most losses to bare soil evaporation. Soil profile attributes in the locations chosen for construction will limit losses to drainage of moisture. Control over moisture loss to runoff and control over addition by overland flow and wind blown snow will result in incorporation of significant quantities of additional water into the soil profile. These results indicate that cobble-bordered gravel-mulched plots do comprise an irrigation technology.