Natural Ventilation Models and Production Management in an Experimental High Tunnel

High tunnels (HTs) help producers to be more profitable through crop protection and extension of growing season. Proper ventilation and heat managements inside HTs are crucial for adjusting inner microclimates and future obtaining marketable crop production. This study first analyzed daytime ventilation rates that responds to the changes of external weather conditions in a Gothic-type HT located in eastern Tennessee. A mathematical energy balance model and instrumental measurement serve as validation data were developed with a good agreement for ventilation rates (R²>0.70). The coupling of energy balance model and air-flux calculation based on external weather parameters can be considered a valuable method to predict the interior microclimate condition and also can be used to estimate door opening levels. Also, this study using an advanced artificial neural network (ANN) discovers a better structure of HT ventilation models, and input sensitivity revealed that relative humidity and wind direction had the least significant impact on the prediction of inside air temperature. The result shows that ANN is more strongly effective than the previous theoretical model in the estimation of HT ventilation rates and opening levels.

Moreover, A three-year experiment assessed the beneficial effects of independent and combined practices on temperature changes inside the HTs, and further examined the impact of temperature changes on crop production. Specific practices included surface mulches, row covers, cover crops, and irrigation water types. Statistical differences were analyzed in a randomized complete block design for total marketable yields of spring pepper and fall lettuce, as well as the changes in soil and air temperature. Black polyethylene mulch is highly recommended for pepper and lettuce production in HTs. Unfortunately, the benefit of cover crop to improve soil health was not found in this study, although there was a positive soil temperature rise surrounding plant root-zones caused by cover crops. Moreover, row covers continuously added protection from freezing during the cold nights resulting in a higher crop production. Stored rainwater was warmer than the city water except in the cold period, and rainwater irrigation is tended to increase the overall production of pepper but lettuce yielded better by using city water.