A HYDROLOGIC MODEL FOR WATER LEVELS IN THE MAYA RESERVOIRS OF CENTRAL TIKAL

The Classic Maya city of Tikal, in the Petén of Guatemala, is characterized by seasonal rainfall and a water table that is at least 100 m below the surface within relatively permeable Cretaceous limestone. Surface water is extremely limited in this karst landscape. In response, the Maya constructed plaster-lined reservoirs within arroyos and quarries. Contributing watersheds for these reservoirs were often ‘paved’ to enhance runoff efficiency (r_eff). Most published research on these reservoirs portrays them as water-filled features with seasonal overflow.

To better understand the hydraulic function of these reservoirs and evaluate their efficacy to retain water, we developed a spreadsheet-based mass-balance model for a system of three reservoirs in central Tikal—the Temple, Palace and Hidden Reservoirs. Considered within this model is the potential recharge to the system including direct precipitation, runoff, and upstream overflow. Also considered are the potential outputs including direct evaporation, leakage, usage rates, and overflow. Reservoir geometry and watershed areas were simplified for each reservoir from available GIS data. Model parameters include: r_eff = 46–69%, liner thickness (b) = 10–12 cm, liner hydraulic conductivity (K) = 10^-8–10^-9 m/s, reservoir usage rate = 1,600 L/day. Annual water-level models were developed for climate data from Tikal that span 2007-2010. Models for each year were developed using two initial water-level conditions: maximum depth and half depth.

Despite using conservative model parameters, water levels in each reservoir end the year at a lower value than the initial condition in every simulation. Lower r_eff and b and higher K result in periods when reservoirs are empty. Drier model years, such as 2009, reveal long periods of limited water. These results suggest: 1) model parameters or reservoir function need re-evaluation, or 2) the precipitation during the Classic Maya met or exceeded wetter years in the current record.