Abstract:

Drought is one of the natural disasters with the biggest losses and affecting most persons. In the land surface process, water usually transports from soil into plant and atmosphere in the daytime and then it partly reflows into soil at night. This water transporting process in soil, vegetable and atmosphere can be considered as a circuit and the three components of soil, vegetable and atmosphere linked with water flows comprise an open system in representation called circulation system. The meteorological drought, hydrological drought and agricultural drought except socioeconomic drought can be studied in a whole by using it, and drought is an external reflection of its inner contradiction movement of water. After the detailed analysis of the system, it shows that inner process, outer process and temperature-controlling  process is three important mechanisms to support the circulation system. Next, the thermal and dynamical water equations for the system and its each component, comprising three physical variables which are water content, water flux and heat quantity, are deduced in an uniform form. The equations for normal-state without drought or waterlogging and for abnormal-state with drought or waterlogging are deduced too. Finally, the theoretical analysis are also carried out to analyze the drought under the specified conditions such as constant water content, constant water flux, bare land without vegetable, land covered with dense vegetable and ceased water flows. The results are as follows:(1) For arid and semi-arid areas or bare land without vegetable, the trends to emerge drought depend on the signs of  which means water extraneous source differing from its normal-state value. (2) For wet area or dense-vegetable covering area, the trends to appear drought depend on signs of both  and  meaning the time partial derivative of water content difference from the normal state. At last, the building of drought index is discussed simply. In short, the drought can be systematically and quantitatively studied with the circulation system and thermal and dynamical water equations, which has an important significance for the basic theoretical search on drought, drought numerical model and drought’s forecast.

Key words:  drought, circulation system, inner and outer process, thermal and dynamical water equations of drought

摘要：

干旱是一种损失最大、影响人口最多的自然灾害。陆面过程中，白天水分通常从土壤输送到植被与大气中，夜间相反，水分又部分回流到土壤中。这种水分在土壤、植被和大气之间的流动可以看作是一个回路，而构成该回路的土壤、植被和大气3个部件通过水分的纽带作用构成了一个表象上的完整开放系统，称为回路系统。由此系统可以对气象干旱、农业干旱和水文干旱进行统一研究，干旱是该系统内在矛盾运动状况的外在反映。对该回路系统的分析表明，该系统主要通过内外2个过程与温度控制过程维持。基于含水量、水分流量和热量3个物理量，给出回路各部件与系统统一的水分热动力学方程组，并给出描述无旱涝过程的正态方程与描述旱涝过程的差量方程。最后，在含水量不变、流量不变、无植被的裸地、植被覆盖密集的地方以及水分运动停止等特殊情况下对干旱进行理论分析。结果表明：(1)对于干旱半干旱地区或无植被的裸地，干旱发生与否取决于水分源差量S′的符号；(2)对于湿润区或植被覆盖度很大的地区，干旱发生与否取决于S′和W′/t的符号，其中W′/t是含水量差量的时间偏导数。随后简单讨论了干旱检测的问题。总之，以回路系统和水分热动力学方程组可以对干旱进行系统和定量研究，这对于干旱的基础理论研究、干旱模式以及干旱的预测具有重要意义。

关键词: 干旱, 回路系统, 内外过程, 水分热动力学方程组