Assessment of buried drip irrigation in soil of slope-protection vegetation from evapotranspiration perspective

Abstract: A vegetation concrete protection has been one of the most widely used technologies for the bare steep slopes in the ecology fields. Among them, the vegetation evapotranspiration can determine the regional distribution of available water resources in the soil-vegetation-atmosphere system. It is also a high demand to evaluate the utilization of water resources for the high-steep slope. In this study, a systematic evaluation was made of the ecological benefit of herbs planted in vegetation concrete under the buried drip irrigation in soil. An improved Penman-Monteith formula was also adopted to calculate the actual crop evapotranspiration (ETc) and the crop coefficient for the herbs, including Lolium perenne L., Festuca arundinacea, Poa annua L. and the mixed-grass seeds of "Lolium perenne L.+Festuca arundinacea+Poa annua L." Hence, the experimental materials were prepared, and the slope planting soil was selected as the composite artificial soil. Subsequently, the improved Penman-Monteith formula was established for the slope ecological restoration using the slope coefficient factor, according to the project requirement of ecological slope protection. Specifically, the test grass was sown in the surface layer of the proportioned vegetation concrete that was attached to the slope model, when installing the irrigation equipment. The irrigation system was also designed to ensure the same amount of irrigation per day using the average evapotranspiration of plants and irrigation water utilization coefficient. Moreover, the evapotranspiration test was conducted to balance the water resource for the daily vegetation evapotranspiration to measure. In addition, the equipment for buried drip irrigation in soil and slope sprinkler irrigation were installed on two slope models in fields. The results revealed that the actual evapotranspiration and crop coefficient of Festuca arundinacea were the largest, followed by Lolium perenne L. and the lowest of Poa annua L. All the determination coefficients R2 were not less than 0.847 for the linear fitting of the actual crop evapotranspiration and crop coefficient. There was a small difference in evapotranspiration of each plant at the initial stage of the maintenance, particularly at around 4.2 mm/d. After finishing maintenance, the largest and lowest evapotranspiration were obtained for the mixed grass, and Lolium perenne L., which were about 6.2 and 5.7 mm/d, respectively. In the first 20 days of the maintenance stage, there was no significant difference in the actual evapotranspiration of Lolium perenne L., Festuca arundinacea and mixed grass, except Poa annua L. More importantly, the Lolium perenne L., Poa annua L. and mixed grass greatly varied after 20 days, but there was no significant difference in the evapotranspiration of each grass after 30 days. The differences in the evapotranspiration of four herbs increased with time. Particularly, the evapotranspiration of mixed grass was lower than those of Lolium perenne L. and Festuca arundinacea at the early stage, but rose significantly at the later stage. The crop coefficient of mixed grass was also greater than those of the rest of single grass over 50 days. Taking the 30th day (one month) as a threshold, there was only a little divergence of crop coefficient within 30 days. Moreover, there was the largest variation of crop coefficient at the initial stage of growth, indicating a downward trend for the monthly increase. However, the values of crop coefficient were similar for the Lolium perenne L. and Festuca arundinacea at any period. As such, the obtained crop coefficients were utilized to calculate the vegetation ecological value for the evaluation of the ecological benefits of the technology of buried drip irrigation in soil and slope sprinkler irrigation. The results showed that the vegetation ecological value increased with the maintenance time, indicating a positive impact of two technologies on the value. Additionally, the slope sprinkler irrigation presented a higher vegetation ecological value than the buried drip irrigation in soil for the vegetation growth in the early stage of the slope model test. Nevertheless, the buried drip irrigation in soil was superior to the ecological benefit during the middle stage. In a word, the vegetation ecological benefit of buried drip irrigation in soil was better than that of slope sprinkler irrigation, where the ecological value was 40.7%-1 444.0% higher than that of slope sprinkler irrigation.